Objection 1. It would seem that contrition cannot take away the debt of punishment entirely. For satisfaction and confession are ordained for man's deliverance from the debt of punishment. Now no man is so perfectly contrite as not to be bound to confession and satisfaction. Therefore contrition is never so great as to blot out the entire debt of punishment.
Objection 2. Further, in Penance the punishment should in some way compensate for the sin. Now some sins are accomplished by members of the body. Therefore, since it is for the due compensation for sin that "by what things a man sinneth, by the same also he is tormented" (Wisdom 11:17), it seems that the punishment for suchlike sins can never be remitted by contrition.
Objection 3. Further, the sorrow of contrition is finite. Now an infinite punishment is due for some, viz. mortal, sins. Therefore contrition can never be so great as to remit the whole punishment.
On the contrary, The affections of the heart are more acceptable to God than external acts. Now man is absolved from both punishment and guilt by means of external actions; and therefore he is also by means of the heart's affections, such as contrition is.
Further, we have an example of this in the thief, to whom it was said (Luke 23:43): "This day shalt thou be with Me in paradise," on account of his one act of repentance.
As to whether the whole debt of punishment is always taken away by contrition, this question has already been considered above (Sent. iv, D, 14, 2, 1,2; III, 86, 4), where the same question was raised with regard to Penance.
I answer that, The intensity of contrition may be regarded in two ways. First, on the part of charity, which causes the displeasure, and in this way it may happen that the act of charity is so intense that the contrition resulting therefrom merits not only the removal of guilt, but also the remission of all punishment. Secondly, on the part of the sensible sorrow, which the will excites in contrition: and since this sorrow is also a kind of punishment, it may be so intense as to suffice for the remission of both guilt and punishment.
Reply to Objection 1. A man cannot be sure that his contrition suffices for the remission of both punishment and guilt: wherefore he is bound to confess and to make satisfaction, especially since his contrition would not be true contrition, unless he had the purpose of confessing united thereto: which purpose must also be carried into effect, on account of the precept given concerning confession.
Reply to Objection 2. Just as inward joy redounds into the outward parts of the body, so does interior sorrow show itself in the exterior members: wherefore it is written (Proverbs 17:22): "A sorrowful spirit drieth up the bones."
Reply to Objection 3. Although the sorrow of contrition is finite in its intensity, even as the punishment due for mortal sin is finite; yet it derives infinite power from charity, whereby it is quickened, and so it avails for the remission of both guilt and punishment.
Source: Summa Theologica (New Advent)