What do you say to a repentant person who has suffered severe psychological illnesses because of his belief that his statement about his Lord, "Indeed, Allah did not benefit me," is an act of disbelief from which his repentance will not be accepted? And is his repentance accepted between him and his Lord, or is there no repentance for him?

The mercy of Allah is greater than any sin, and Allah accepts repentance from His repenting servants, no matter how great their sins. Allah Almighty says: "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.'" If you repent and regret, your repentance is accepted, and "the one who repents from sin is like one who has no sin." Have a good opinion of your Lord, cast aside whisperings (waswas), turn to obedience, and do more good deeds, for good deeds erase bad deeds.