Hi, CupOfMilk. I noticed that this problem is still weighing on your mind. (Over the past nine months or so, you have started several threads about sins committed before you became Christian.)
I don’t know whether you are Catholic, or whether you belong to a Protestant denomination. One of the greatest treasures we have in the Catholic Church is the Sacrament of Reconciliation (also called Penance or Confession) in which we are assured of God’s forgiveness, and the priest can give us good advice on how to deal with things that plague us from our past.
Are you Catholic? (Have you been fully initiated into the Catholic Church by Catholic baptism or by being formally received into the Catholic Church as an adult?) If so, I highly recommend that you go to confession to “wipe the slate clean” of all these things that are nagging at your conscience.
If you are not Catholic, perhaps this is God’s way of calling you to the Catholic Church, where you can receive the Sacrament of Reconciliation that assures you of His forgiveness. The problem with your idea to “just ask God for forgiveness and let it be” is that, outside the Sacrament that God has given us as the ordinary means to obtain His forgiveness, we can never be sure that we are truly forgiven. When we go to confession and the priest, acting in the person of Christ, speaks the words of absolution (forgiveness), we can be absolutely certain we are forgiven, because Jesus Himself said so in speaking to the apostles, the predecessors of today’s Catholic bishops and priests: “Whose sins you forgive, they are forgiven them.”
So if you are not Catholic, pray to the Holy Spirit to lead you to the Church Jesus founded, and talk to a Catholic priest about how to make it happen.