He is complaining that he does not feel reassurance in his heart when remembering Allah, may He be exalted
Question

What should I do, as I have a problem? Please advise me. I always offer all the prayers in the mosque, and I keep away from sins. I used to say tasbih (“Subhan Allah (Glory be to Allah”)) a great deal, and I tried to draw close to Him and prayed that He would make me one of His righteous slaves and bless me with a heart that trembles with fear of Him. I repeated this supplication approximately three times yesterday, and I spent the night in prayer (qiyam) and offered the same supplication. I thought positively of Allah and was certain of a response. But after spending the night in prayer, then sleeping on the following day, I felt distressed and could not recite tasbih, and I felt no sense of reassurance when remembering Allah. I became very sad and wept with sorrow. I think positively of Allah, but I said to myself: Why is it happening to me that I do not feel any sense of reassurance when remembering Allah? I began to weep and I said to myself: Why? What have I done? I tried to check myself, but I could not remember committing any sin. On the contrary, I remember Allah a great deal, but now I am unable to offer any supplication or recite any dhikr, and I am only praying regularly in the mosque. I have stopped spending the night in prayer (qiyam al-layl). What should I do now?
Praise be to Allah.It should be understood that the more a person tries to draw close to Allah, the more the Shaytan will try to block his path in order to undermine his efforts and make his life difficult, and divert him from worshipping and obeying Allah, may He be exalted.
So the Muslim must strive to be patient and strive against his nafs and evil inclinations and against the Shaytan. He should offer a lot of supplication and beseech Allah, may He be exalted, to help him to remain steadfast.
Upholding regular prayer, keeping away from haram things, remembering Allah a great deal, and seeking to draw close to Him by doing supererogatory (nafil) acts of worship – all of this is the straight path, which you must strive hard to adhere to and be patient in bearing its hardships.
Remember the words of the Messenger (blessings and peace of Allah be upon him): “A person will still be answered so long as his supplication does not involve sin or severing ties of kinship, and so long as he does not become impatient.” It was said: O Messenger of Allah, what does being impatient mean? He said: “Saying, I offered supplication, and I offered supplication, and I did not receive any response, then he becomes frustrated and stops offering supplication.” Narrated by Muslim (2735).
So do not say, “I remembered Allah a great deal, and I prayed and I spent the night in prayer, but my heart was not softened.” Rather you should strive hard in remembering Allah (dhikr), praying and spending the night in prayer (qiyam), until you attain what you are seeking.
As it is said: Whoever keeps knocking on the door, it will be opened to him; as for the one who knocks only once and goes away…
You should understand that what befalls you sometimes of grief, sadness, distress and lack of motivation is part of the Shaytan’s scheme that he uses against the believers to make them despondent. Once a person realizes that, and he becomes certain that it is the scheme of the Shaytan, then he will be on the alert and will seek refuge with Allah from that.
And you should understand that the sweetness of faith and finding pleasure in the remembrance of Allah, may He be exalted, is something that the believer will never attain except after striving against his nafs and evil inclinations, and against the Shaytan. Hence one of the early generations said: I struggled to make myself read the Quran for twenty years, then I found pleasure in it for twenty years. Similarly, finding pleasure in the remembrance of Allah and obeying Him can only come after persisting in doing righteous deeds and striving hard for a long time, until the believer starts to taste the sweetness of faith.
May Allah help you to do that which He loves and is pleased with, and may He suffice you against the evil of your own self (nafs) and the evil of the Shaytan.
And Allah knows best.