How does a servant repent and seek forgiveness for sins he does not remember, and what is the ruling on the formula of seeking forgiveness: "I seek forgiveness from Allah for what Allah knows. I seek forgiveness from Allah Almighty, besides whom there is no god, the Ever-Living, the Sustainer, and I repent to Him, for the sins I know, and for the sins I do not know"?

No matter how great the sin, Allah's forgiveness is greater. And whoever repents with sincere repentance, Allah will accept his repentance and erase his sin, and he will return as if he had never sinned, for the Prophet, peace and blessings be upon him, said: "The one who repents from a sin is like one who has no sin."

Continue on the path of repentance and uprightness, and increase in good deeds that wipe out sins, for Allah the Almighty says: "Indeed, good deeds do away with evil deeds." And for the sins you do not know about, repent from them with a general repentance.

The seeking of forgiveness mentioned in the question is good, for it was narrated from the Prophet, peace and blessings be upon him: "Whoever says: 'I seek forgiveness from Allah, the Magnificent, whom there is no god but He, the Ever-Living, the Sustainer, and I repent to Him,' his sins will be forgiven, even if he had fled from the battlefield." It is also recommended to frequently recite other authentic formulas of seeking forgiveness, such such as the master of seeking forgiveness (Sayyid al-Istighfar).