Why does the questioner feel disturbed and experience irregular heartbeats when not praying Fajr in congregation, and feel tranquility when praying it, and is this a grace from Allah or a whisper from Satan?

Performing Fajr prayer in congregation has immense virtue; whoever prays it is under the protection of Allah. The feeling of comfort and tranquility after performing it is due to performing good deeds that enlighten the heart and gladden the soul, for obedience brings pleasure, vitality, the coolness of the eyes, and the sweetness of nearness to Allah. Prayer in general brings tranquility because it is part of the remembrance of Allah, by which hearts are reassured, as the Prophet, peace and blessings be upon him, said: "Relieve us with it, O Bilal." Therefore, strive for steadfastness and to maintain prayers in congregation in the mosque, and do not pay attention to the whispers of Satan that discourage you from obedience or instill arrogance in you.