Can a supplication be accepted in which it was said: "O Allah, grant me mountains of sins and overlook my good deeds," and how can it be nullified?

What was uttered by your tongue in the form of a wrongful prayer against yourself carries no accountability, due to the lack of intention. Allah has indeed overlooked errors, forgetfulness, and that which one is coerced into. Some scholars are of the view that a person's prayer against themselves, even if intentional, is considered vain and will not be answered, based on the Almighty's saying: "And if Allah were to hasten for the people the evil [they invoke] as He hastens for them the good, their term would have been concluded for them." This indicates His clemency and gentleness towards His servants, and His non-response to their prayers against themselves, their wealth, or their children when in distress and anger; for He knows they do not intend evil, unlike His response to their prayers for good and blessings.