Psalm 19:12: “But who can discern their own errors? Forgive my hidden faults.”
Nobody can really forgive his own sins. You can try to do that, but it means you only forgive yourself, but by this act you do not put away legal guilt that hangs on you for doing this or that specific sin. Who can do that then? Only God! He is the One who can forgive or not forgive. I am so happy that He did forgive us in Christ. The only thing that you have to do now is to accept this gift. Accept His gift of forgiveness and be forgiven forever. Even hidden faults will be forgiven.