If you and I had to remember every sin we ever committed before God would forgive us, none of us could be saved. Sin is like a deadly disease that infects everything we do, and often we are blinded to our own sin. Our memories are imperfect also, and we'll never remember everything we did wrong.
After all, we don't just sin when we do something we know is wrong. We also sin when we fail to do something right -- act kindly toward someone; give to help people in other parts of the world who are starving; and so forth. We sin as well when we do things from wrong or selfish motives (although we may not even be aware of it at the time). Most of all, we sin when we do not give God His rightful place in our lives, and instead turn our backs on Him and His will. This is why the Bible says, "There is no one righteous, not even one" (Romans 3:10).
But listen: When Jesus Christ went to the cross, He took upon Himself every sin we ever committed -- even the ones we can't remember. He was without sin, but all our sins were placed in Him, and He took the punishment and death we deserved. The Bible says, "As far as the east is from the west, so far has he removed our transgressions from us" (Psalm 103:12).
By faith ask Jesus Christ to come into your life and cleanse you of all your sins -- and He will. Then ask Him to help you walk with Him and live for Him every day.