Tips on How to Remain Steadfast in Your Faith
Question

I have a problem and a question: Allah has guided me months ago. But I started to relapse with time. I do not know why. My Iman started to decrease gradually until I left prayers and became weak.
But, Alhamdulillah, I tried not to fall into sins. But I missed many prayers while sleeping or away. By time I left prayers totally for days. I feel I have a complex regarding praying. I miss many prayers; I feel I will not get rid of this complex. What is the solution and what shall I do?
Praise be to Allah.Be thankful to Allah
You should appreciate the blessing that Allah has bestowed upon you by enabling you to repent and adhere to your Religion, before death comes suddenly. So you should thank Him greatly for this blessing , which should make you strive harder to obey Him.  
Al-Bukhari (4836) and Muslim (2819) narrated that al-Mughirah ibn Shu’bah said: The Prophet (peace and blessings of Allah be upon him) stood (in prayer) until his feet became swollen. It was said to him: Allah has forgiven your past and future sins. He said: “Should I not be a thankful slave?” 
Seek refuge with Allah from a bad situation after a good situation
So how can you accept to respond to goodness with something bad, and to go back to square one after you had gone so far in the way of righteousness? Rather we hope that you will go back to the way you were when you started to follow the straight path, for you have deviated and gone astray from the path after Allah blessed you with righteousness. 
The Prophet (peace and blessings of Allah be upon him) used to seek refuge with Allah from such a thing. In Sahih Muslim (1343) it is narrated that ‘Abd-Allah ibn Sarjis said: When the Messenger of Allah (peace and blessings of Allah be upon him) travelled, he would seek refuge with Allah from the hardships of travel, from bad consequences, from a bad situation after a good situation, from the prayer of one who has been wronged, and from an ill-fated outcome with regard to family and wealth. 
In His Noble Book, Allah strikes a similitude for the one who goes back to bad ways and destroys what he has built up, and turns away from the path of guidance, which explains the bad situation he is in and what he has chosen for himself, and He warns His slaves against doing this foolish deed: 
“And fulfil the Covenant of Allah (Bai‘ah: pledge for Islam) when you have covenanted, and break not the oaths after you have confirmed them — and indeed you have appointed Allah your surety. Verily, Allah knows what you do. And be not like her who undoes the thread which she has spun, after it has become strong, by taking your oaths as a means of deception among yourselves, lest a nation should be more numerous than another nation. Allah only tests you by this (i.e. who obeys Allah and fulfils Allah’s Covenant and who disobeys Allah and breaks Allah’s Covenant). And on the Day of Resurrection, He will certainly make clear to you that wherein you used to differ (i.e. a believer confesses and believes in the Oneness of Allah and in the Prophethood of Prophet Muhammad which the disbeliever denies and that is their difference amongst them in the life of this world).” [al-Nahl 16:91-92]
Shaykh Ibn al-Sa’di (may Allah have mercy on him) said: 
“This includes everything that a person has covenanted to Allah, such as acts of worship, vows and faith, because fulfilling that is righteousness. It also includes that which he and others have covenanted, such as covenants between people.  
“And be not” in breaking the covenants in the worst and most foolish manner, “like her” who spins a strong thread, then when she has nearly finished it, she breaks it and it comes undone, so she exhausted herself spinning the thread and then undoing it, and she did not gain anything but frustration, exhaustion and foolishness. The same applies to the one who breaks the covenant; he is a wrongdoer, ignorant, foolish, lacking in religious commitment and manliness.” (p. 447) 
Giving up prayer is a grave sin 
So hasten to repent from this, for giving up prayer is one of the gravest of sins by which Allah is disobeyed. The Prophet (peace and blessings of Allah be upon him) described not praying as kufr, and he said: “The covenant that stands between us and them is prayer; whoever gives it up is a kafir.” Narrated by al-Tirmidhi (2545) and by Ahmad and others; classed as sahih by al-Albani. 
It is strange how the brother can say: I have tried not to fall into sin,” then he says that he missed a lot of prayer. What is his concept of sin in that case?! 
Giving up prayer is the greatest and most serious sin. You have tried to prevent yourself from doing every sin except associating others with Allah (shirk). 
So hasten to repent and regret what you have done, before there comes a moment when man regrets at the time when regret is to no avail. 
Do not help your enemy (Satan) against yourself
As for you saying that you have a “complex about prayer” and you feel that you will never recover from this complex, this is a trick of the shaytan who makes this idea seem attractive. You are the one who has helped your enemy against yourself, and you let him fill your mind with the idea of this complex which stems from illusions and laziness, and lack of resolve to do good. So hasten to solve this problem by obeying Allah, may He be exalted, and keeping your wudu, and hastening to pray when the time for prayer begins. 
It was narrated from Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: “The Shaytan ties three knots at the back of the head of any one of you when he goes to sleep, striking each knot and saying: “You have a long night ahead, so sleep.” If he wakes up and remembers Allah, one knot is undone. If he does wudu, two knots are undone. If he prays, all the knots are undone, and he starts the day energetic and in a good mood. Otherwise, he starts his day in a bad mood and feeling lazy.” (Narrated by al-Bukhari (3269) and Muslim (776) 
Ibn ‘Abd al-Barr (may Allah have mercy on him) said: 
“This hadith indicates that the shaytan causes a person to sleep and makes him feel heavy and lazy through his efforts and waswas, and his ability to tempt, mislead and make falsehood seem attractive, except the sincere slaves of Allah. 
This hadith indicates that the shaytan may be expelled by means of remembrance of Allah (dhikr), as well as wudu and prayer.” (Al-Tamhid (19/45)
Ibn al-Qayyim (may Allah have mercy on him) said: 
“Undoubtedly prayer itself offers good health to the body, and helps the body to rid itself of impurities and keep that which is the most beneficial thing for it, as well as offering protection to one’s faith and happiness in this world and the Hereafter. 
Similarly, qiyam al-layl is one of the most beneficial means of protecting one’s well-being and preventing many chronic diseases, and it is one of the most energizing things for the body, soul and heart, as it says in al-Sahihayn… and he quoted the hadith.” (Zad al-Ma'ad, 4/225)
Seek the help of Allah and do not feel helpless
Do not feel weak before your enemy, and do not let him gain any power over you. Seek the help of Allah and do not feel helpless – as your Prophet (peace and blessings of Allah be upon him) has enjoined you. Remember that “ever feeble indeed is the plot of Shaytan (Satan)” [al-Nisa 4:76]. Remember that the obligatory prayers are easy and the Muslim will not find it burdensome to perform them regularly. 
Keep good company
With regard to what you mention about losing interest, the reason for that may be the kind of people with whom you are keeping company. Try as much as you can to attend gatherings of dhikr and knowledge, and to sit with good companions, for acts of worship are easy when a person sees others whom he can take as examples and who can help him with it. 
A person will inevitably be exposed to situations which make him feel stronger and more keen to do good, and other situations where that keenness will be less. But it is not permissible to let that lead you to giving up obligatory duties or committing haram actions. 
The Prophet (peace and blessings of Allah be upon him) said: “Every deed has a period of enthusiasm, and every period of enthusiasm is followed by a slackening off. If a person’s enthusiasm is for my Sunnah, then he has succeeded, but whoever chooses something else when he slackens off is doomed.” (Narrated by Ahmad (6725) and classed as sahih by al-Albani in Sahih al-Jami’ (no. 2151) 
Set a regular program for yourself
Hence our advice to you is to set a regular program for yourself which you should not neglect, including the obligatory and confirmed nafl prayers. If you do more than that occasionally that will be good upon good, and if you fall short then do not fall short in doing the obligatory prayers. 
How to make up missed prayers
With regard to the prayers that you have missed in the past , whatever you missed because of sleep, there is no sin on you for that but you have to make them up . As for those that you missed with no excuse, i.e., you were too lazy to do them before the time for them ended, then you have to repent, and in that case making them up will not avail you anything. What you have to do is offer a lot of nawafil prayers and pray for forgiveness, so that Allah may forgive you. 
And Allah knows best.