What should one do who has committed a grave sin and repented with a repentance that he does not see as sufficient for the forgiveness of the sin?

I thank Allah for saving you from sin and guiding you to repent from it. Know that sincere repentance erases all previous sins, even disbelief and major shirk (definition). Allah has promised to accept the repentance of His servants and to forgive their evil deeds, as stated in the noble verses: "And it is He who accepts repentance from His servants and pardons sins" (Ash-Shura: 25), and "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins.'" (Az-Zumar: 53). And among Allah's favors upon the repentant is that He transforms their evil deeds into good deeds, as stated in His saying: "Except for those who repent, believe, and do righteous deeds - for them Allah will change their evil deeds into good ones" (Al-Furqan: 70). You must remain steadfast in obedience to Allah, do good deeds, and continuously renew your repentance.