Jesus has given us a gift, called Reconciliation. This takes away sin, and begins the restoration, but it does not remedy all the hurt sin has caused to the heart and soul. That is why we do Penance for our sins. Like paying for the broken window, once you've said your were sorry. You are forgiven, but you must make amends.
Penance is both an act of contrition, and an act of devotion.
This makes the person stronger, and better able to resist sin in the future, as well as answering to the needs of justice. And most importantly, penance opens us to the grace and strength of the Holy Spirit. One particularly powerful form of Penance, is to perform one of the Acts of Mercy. These bestow blessings and open us to grace whenever we do them. Use them to strengthen the soul whenever you sin, turning to the Holy Spirit, and asking for His grace to come upon you.