Is the servant's feeling that Allah will not accept his repentance, prayers, and good deeds due to his past sins a correct feeling or from the whispers of Satan?

If a Muslim repents with a sincere repentance, then repentance erases what came before it, even for a disbeliever, his repentance erases his disbelief that came before it, as Allah says: (Say to those who disbelieved, if they cease, what has passed will be forgiven for them). The penitent's feeling of the enormity of the sin is positive if it leads him to good deeds and seeking refuge in Allah, and perhaps a sin led to Paradise. But if this feeling leads to anxiety and despair, then it is from the stratagems of Satan. For Allah accepts repentance from His servants no matter how great their sins, and He has called them to repentance and made them desire it, and He loves those who repent. He said: (Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.") What is obligatory is sincere repentance and trust in Allah's promise, and to proceed with righteous deeds without paying attention to the whispers of Satan.