Objection 1. It would seem that man should be contrite on account of the punishment, and not only on account of his sin. For Augustine says in De Poenitentia [Cf. Hom. 50 inter 1]: "No man desires life everlasting unless he repent of this mortal life." But the morality of this life is a punishment. Therefore the penitent should be contrite on account of his punishments also.
Objection 2. Further, the Master says (Sent. iv, D, 16, cap. i), quoting Augustine (De vera et falsa Poenitentia [Work of an unknown author]), that the penitent should be sorry for having deprived himself of virtue. But privation of virtue is a punishment. Therefore contrition is sorrow for punishments also.
On the contrary, No one holds to that for which he is sorry. But a penitent, by the very signification of the word, is one who holds to his punishment ["Poenitens," i.e. "poenam tenens"]. Therefore he is not sorry on account of his punishment, so that contrition which is penitential sorrow is not on account of punishment.
I answer that, As stated above (Question 1, Article 1), contrition implies the crushing of something hard and whole. Now this wholeness and hardness is found in the evil of fault, since the will, which is the cause thereof in the evil-doer, sticks to its own ground*, and refuses to yield to the precept of the law, wherefore displeasure at a suchlike evil is called metaphorically "contrition." [There is a play on the words here--'integer' (whole) and 'in suis terminis' (to its own ground)]. But this metaphor cannot be applied to evil of punishment, because punishment simply denotes a lessening, so that it is possible to have sorrow for punishment but not contrition.
Reply to Objection 1. According to St. Augustine, penance should be on account of this mortal life, not by reason of its mortality (unless penance be taken broadly for every kind of sorrow); but by reason of sins, to which we are prone on account of the weakness of this life.
Reply to Objection 2. Sorrow for the loss of virtue through sin is not essentially the same as contrition, but is its principle. For just as we are moved to desire a thing on account of the good we expect to derive from it, so are we moved to be sorry for something on account of the evil accruing to us therefrom.
Source: Summa Theologica (New Advent)