He is assailed by doubts because he no longer focuses on his prayer
Question

I hope that you can tell me how I can make myself enjoy and like obeying Allah and doing acts of worship. Every time I see foreigners who become Muslim, and how Allah has sent down tranquillity to them, I weep bitterly, because I want to be like them. Even when I listen to lectures and stories of the Prophets, and I listen to Qur’an, I weep and think of death and meeting Allah. I weep, but one or two hours after listening to Qur’an or religious lectures, I go back to my old ways, and I start taking prayer and many other matters lightly. I begin to doubt, thinking that I am drifting away from my religion and that I am going to apostatise. Many doubts come to my mind, and I cannot rid myself of these thoughts. A while ago, when I used to go out with my friends, I could not wait to come back home and pray two rak‘ahs, because prayer was very dear to me. Now I want to go back to the way I used to be, but I cannot. Even when I pray, I rush my prayer. I have begun to collapse. I am certain that the religion of Islam is the true religion, and I am afraid that I might die when there is some doubt in my heart, and my faith and deeds will not be accepted from me.
Praise be to Allah.It is clear to us from your question that you are suffering from whispers of exaggeration, for you do not only exaggerate about problems and make them seem complex, rather you have reached the point where you think that this will be the cause of your collapse, as you put it, and the point of a real psychological crisis that would prevent you from striving, and that has brought you down to a state of utter despair.
But, by Allah’s grace, the matter is much easier than that.
You are a Muslim, first and last, praise be to Allah. You believe in the six articles of faith; you believe in Allah, His angels, His Books, His Messengers, the Last Day, and the divine decree, both good and bad. You are striving to adhere to the five pillars of Islam, the first of which is the twin testimony of faith, and you are carrying out your religious duties, such as prayer, giving zakaah, fasting and Hajj.
All of that should help you to attain a high status before Allah, may He be glorified, for we all believe in His Oneness and the messages that He sent down, and we testify to the Prophethood of our Prophet Muhammad (blessings and peace of Allah be upon him); we declare our commitment to following him and adhering to his path. So why do you not think of all these achievements and certain truths that are of the greatest significance before Allah, may He be glorified?
Are we not the ummah of Muhammad (blessings and peace of Allah be upon him)? And he promised us that everyone who says Laa ilaaha ill-Allah (there is no god but Allah) sincerely from the heart will enter Paradise. Have we not heard the hadith narrated by Abu Dharr (may Allah be pleased with him) from the Prophet (blessings and peace of Allah be upon him), that he said: “That was Jibreel who appeared to me beside the harrah and said: ‘Give the glad tidings to your ummah that whoever dies without having associated anything with Allah will enter Paradise.’ I said: ‘O Jibreel, even if he committed theft or zina?’ He said: ‘Yes.’ I said: ‘Even if he committed theft or zina?’ He said: ‘Yes, and even if he drank alcohol.’” Narrated by al-Bukhaari (6443).
You – praise be to Allah – are regular with your prayer and worship, but you feel that you are falling short in your performance, as you are not able to humble yourself and focus, and to perform the prayer properly. This is something that happens to most Muslims. What is required of them is to persist in striving hard, so as to reach the point of finding joy in worship, which all the righteous strive to attain. They should follow the example of Thaabit al-Banaani (may Allah have mercy on him) who said: I pushed myself to stand up and pray for twenty years, then I found joy in it for twenty years. Narrated by Abu Nu‘aym in Hilyat al-Awliya’ (2/320).
Striving and persisting in worship, and trying one’s utmost to do it properly and attain sincerity and focus of mind therein, is the main concern of the righteous and pious. No one claims to have attained perfection in that regard, but at the same time, no one who is concerned about this matter should despair, for Allah, may He be glorified and exalted, does not like those who despair of His grace and mercy.
“And who despairs of the mercy of his Lord except for those astray?” [al-Hijr 15:56].
And He loves His slaves who hope for His mercy and grace. By the vastness of His kindness, He forgives bad deeds, pardons mistakes, and overlooks shortcomings. Indeed, He, may He be glorified and exalted, says (interpretation of the meaning):
“Except for those who repent, believe and do righteous work. For them Allah will replace their evil deeds with good. And ever is Allah Forgiving and Merciful.
And he who repents and does righteousness does indeed turn to Allah with [accepted] repentance” [al-Furqaan 25:70-71].
Ibn Qayyim al-Jawziyyah (may Allah have mercy on him) said: Being calm when doing acts of worship helps one to have focus of mind and to humble oneself before one’s Lord, as does lowering one’s gaze and focusing with all one’s heart on Allah, may He be exalted, so that one will be able to do this act of worship wholeheartedly and mindfully. Being able to focus and feel humble when praying is the result of this calmness, and physical stillness is the outcome of focus of the heart. The Prophet (blessings and peace of Allah be upon him) saw a man fidgeting with his beard whilst praying, and he said: “If his heart was focused, then his body would be still.”
If you were to say: How could one achieve that?
I say: The way to achieve it is by constantly thinking of your Lord, may He be glorified and exalted, until it is as if you can see Him. That will make you develop a sense of haya’ (shyness, modesty), which will make you feel shy lest you commit a sin, as well as tranquillity, love, submission, humility, and fear and hope, that cannot be attained otherwise. Thinking of Allah constantly watching is the basis for all good deeds of the heart (that is, emotions, feelings, thoughts, intentions, etc.), and is the foundation on the basis of which acts of worship become sound. The Prophet (blessings and peace of Allah be upon him) summed up all the deeds of the heart, both major and minor, in one phrase, when he said, defining ihsaan: “It is to worship Allah as if you see Him.” So think of every religious duty or righteous deed that a person does in the proper manner, and every good deed of the heart (such as positive feelings and emotions), and you will find that this phrase is the basis thereof.
The point is that the individual needs to develop tranquillity and calmness when faced with negative ideas and whispers concerning the basis of faith, so that he may remain steadfast and not deviate, and when faced with thoughts and whispers that could undermine the fundamentals of faith, lest they grow stronger and cause distress and grief, or they become strong and cause him to start thinking of drifting away, which could nullify his faith.
He also needs tranquillity when there are reasons for fear and worry of various kinds, so that his heart may remain steadfast and avoid panic.
And he needs tranquillity when there is cause for joy, lest he overstep the mark that should not be overstepped, which could cause his joy to turn to grief and sorrow.
End quote from I‘laam al-Muwaqqi‘een (4/155-156).
As you are keeping up with prayers on time, trying hard to develop focus, and striving to humble yourself before Allah in prayer and venerate Him, you should know that by doing so, you are doing the best and most beloved to Allah of deeds, even if the movement of your tongue and your physical actions are lacking in focus and even if you are not able to feel a connection with Allah, may He be glorified. Perhaps Allah, Who is aware of what you are going through of worry and how sincerely you are striving to attain the level of calmness and tranquillity in worship that you seek, will bless you with it and will decree it for you, as a reward for your striving and for your pain and hopes.
Al-Ghazaali (may Allah have mercy on him) said:
Seeking forgiveness verbally (without focus of mind) is still a good deed, because moving the tongue to say these words, even without focus, is better than moving the tongue at that moment in backbiting a Muslim or engaging in idle talk. Indeed, it is better than not saying anything. The virtue of uttering istighfaar (prayer for forgiveness), even without focus, when compared to not saying anything, is better, but it still falls short of the proper way of saying it, which is with focus of mind.
Hence one of them said to his shaykh, Abu ‘Uthmaan al-Maghribi: Sometimes my tongue moves with recitation of dhikr or Qur’an, but my heart is not focused on what I am saying.
He said: Give thanks to Allah, for He is causing you to use one of your faculties for something good and making it become accustomed to dhikr, and He has not caused you to use it for some evil purpose or let it get used to idle talk.
What he mentioned is true, for if the physical faculties get used to doing good until it becomes second nature, that will ward off many sins from you. If a person’s tongue gets used to speaking words of istighfaar, when he hears someone else speaking a lie, his tongue will utter what it is used to saying, so he will say: Astahfir-Allah (I seek Allah’s forgiveness), whereas the one who is used to engaging in idle talk, his tongue will utter words such as: How foolish you are, or what a liar you are.
This is referred to in the verses in which Allah, may He be exalted, says (interpretation of the meaning):
“indeed, Allah does not allow to be lost the reward of those who do good” [Hood 11:115]
“while if there is a good deed, He multiplies it and gives from Himself a great reward” [an-Nisa’ 4:40]. 
See how Allah multiplies it and brings about many benefits, as He has made istighfaar recited habitually, without focus, a means of warding off the sins of backbiting, cursing and idle talk.
This is bringing about many benefits in this world for the smallest act of obedience; the benefits and reward in the hereafter would be much greater, if only they knew.
Beware of becoming obsessed with your shortcomings when doing any act of worship, lest that make you lose interest in doing acts of worship altogether, for this is a trick of the Shaytaan that he uses to deceive people and make them think that they are people of deep insight who are able to notice subtleties and examine what is in the heart, and to say, What good is there in saying words of dhikr with the tongue when our hearts are not focused?
With regard to this devilish trick, people fall into three categories: those who wronged themselves, those who are moderate, and those who are foremost in good deeds.
The one who is foremost in good deeds says: You are right, O cursed one! But it is a correct statement intended for a wrong purpose, and I shall certainly punish you twice and annoy you on two counts: in addition to moving my tongue, I shall start focusing in my heart too. Thus he is like one who wants to treat the wound of the Shaytaan by rubbing salt in it.
As for the one who wronged himself and is deceived, he feels proud of himself for noticing this subtle matter, but he is unable to perfect his deeds, so he gives up the habit of reciting dhikr altogether. Thus he goes along with the Shaytaan and accepts his deceiving argument, and there is complete harmony between them.
As for the one who is moderate, he is not able to annoy the Shaytaan by  making himself focus on what he is doing of uttering words of dhikr and doing acts of worship, and he is also aware of his shortcomings, for he was uttering words of dhikr and doing acts of worship without being focused. Yet at the same time he was aware of the fact that uttering words of dhikr (even without being focused) is better than remaining silent or engaging in idle talk, so he persisted in it and asked Allah, may He be exalted, to help him so that he would be able to focus every time he uttered words of dhikr or did acts of worship.
So the foremost in doing good is like the weaver who was criticised for being a weaver, so he gave it up and became a scribe [which was regarded as a superior profession].
The one who wronged himself and falls behind is like the one who gave up weaving altogether, and became a sweeper.
The one who is moderate is like the one who is unable to be a scribe, and said: I understand that the profession of a weaver is not good; the weaver in comparison to the scribe is not good, but in comparison to the sweeper he is good, so even if I am unable to be a scribe, I shall not give up being a weaver.
This is how you should understand criticism and praise, for these matters are to be judged in comparison with other things and should not be judged in isolation. You should not regard the smallest acts of worship or sins as being insignificant. End quote from Ihyaa’ ‘Uloom ad-Deen (4/48).
A wise man said: there is no hope without effort.
What we mean is that nothing will bring you out of the state of despair into which the Shaytaan has caused you to fall except bearing in mind what is mentioned above of hoping for Allah’s mercy and following the example of the righteous, starting by adhering to prayer on time, doing wudoo’ properly beforehand, being deliberate in recitation in the prayer, and trying to focus when reciting the adhkaar after the prayer. Do not forget to recite the supplication following every prayer: “Allahumma a‘inni ‘ala dhikrika wa shukrika wa husni ‘ibaadatika (O Allah, help me to remember You, give thanks to You, and worship You properly).”
On this website you will find previous answers that speak of losing interest in worship, and the causes and remedy thereof. Please see questions no.  47565 and 70314.
There are also answers that speak of doubts and whispers: 39684 and 21052.
And Allah knows best.