Penance forgives any sins committed after Baptism. But if you are not repentant, then you are not forgiven. Only a Bishop or a priest can absolve sins in Confession. An actual mortal sin occurs when one does something that is seriously immoral, with full deliberation and knowledge. To be forgiven, admit your sin in Confession, with true sorrow (regret for sin) and repentance (a turning away from sin, toward the love of God and neighbor). Confession restores the state of salvific grace lost by actual mortal sin.