What is the reason for not feeling humility and comfort when performing acts of worship, especially prayer, despite sincere intention?

Sins remove humility from the heart and increase its darkness, as stated in the Hadith: "Indeed, when a servant commits a sin, a black spot is marked on his heart..."

It is necessary to achieve sincere repentance, be persistent in supplication, not despair of Allah's mercy, and have a good opinion of Him, for Allah Almighty says in the Hadith Qudsi: "I am as My servant thinks of Me, so let him think of Me as he wishes."

The mentioned Hadith about a man who stole and then prayed, and Allah forgave him, has not been found. Theft necessitates repentance and the return of rights, whereas the authentic Hadith relates to someone who kissed [a woman unlawfully] and was forgiven through the blessing of prayer and repentance.