The penance received during confession is intended for two things: restitution for wrongs committed against other people (e.g., returning stolen goods, reconciling with the estranged), and as a token toward restitution for wrongs committed against God that we are able to compensate. (We cannot pay the eternal debt for our sins; that is what Jesus paid on our behalf. But we can pay the temporal debt, and will do so either in this life or in purgatory.)
While the penance given in confession is but a token of the penance we owe for our temporal debt, penitents must be extremely careful about taking up further penances on their own. Without proper spiritual guidance from a balanced, orthodox, spiritually mature confessor and/or spiritual director, a penitent can easily fall into scrupulosity and/or unhealthy spiritual regimens. If you feel called to take up further penances in atonement for sin, find a solid confessor and/or spiritual director first.
