Here's two quick tidbits of information meant to help you grow spiritually:
This, according to St. Alphonsus, when you feel remorse for a sin that has already been confessed, it is a sure sign it has been forgiven. Let it go.
This, according to my priest, (a great confessor!) Always confess one mortal sin from your past each time you go to confession. Even though it has already been confessed and forgiven, doing so helps you to become humble.