Objection 1. It would seem that penance cannot be continuous. For it is written (Jeremiah 31:16): "Let thy voice cease from weeping, and thy eyes from tears." But this would be impossible if penance were continuous, for it consists in weeping and tears. Therefore penance cannot be continuous.
Objection 2. Further, man ought to rejoice at every good work, according to Psalm 99:1: "Serve ye the Lord with gladness." Now to do penance is a good work. Therefore man should rejoice at it. But man cannot rejoice and grieve at the same time, as the Philosopher declares (Ethic. ix, 4). Therefore a penitent cannot grieve continually for his past sins, which is essential to penance. Therefore penance cannot be continuous.
Objection 3. Further, the Apostle says (2 Corinthians 2:7): "Comfort him," viz. the penitent, "lest perhaps such an one be swallowed up with overmuch sorrow." But comfort dispels grief, which is essential to penance. Therefore penance need not be continuous.
On the contrary, Augustine says in his book on Penance [De vera et falsa Poenitentia, the authorship of which is unknown]: "In doing penance grief should be continual."
I answer that, One is said to repent in two ways, actually and habitually. It is impossible for a man continually to repent actually. for the acts, whether internal or external, of a penitent must needs be interrupted by sleep and other things which the body needs. Secondly, a man is said to repent habitually. and thus he should repent continually, both by never doing anything contrary to penance, so as to destroy the habitual disposition of the penitent, and by being resolved that his past sins should always be displeasing to him.
Reply to Objection 1. Weeping and tears belong to the act of external penance, and this act needs neither to be continuous, nor to last until the end of life, as stated above (Article 8): wherefore it is significantly added: "For there is a reward for thy work." Now the reward of the penitent's work is the full remission of sin both as to guilt and as to punishment; and after receiving this reward there is no need for man to proceed to acts of external penance. This, however, does not prevent penance being continual, as explained above.
Reply to Objection 2. Of sorrow and joy we may speak in two ways: first, as being passions of the sensitive appetite; and thus they can no. wise be together, since they are altogether contrary to one another, either on the part of the object (as when they have the same object), or at least on the part of the movement, for joy is with expansion [Cf. I-II, 33, 1] of the heart, whereas sorrow is with contraction; and it is in this sense that the Philosopher speaks in Ethic. ix. Secondly, we may speak of joy and sorrow as being simple acts of the will, to which something is pleasing or displeasing. Accordingly, they cannot be contrary to one another, except on the part of the object, as when they concern the same object in the same respect, in which way joy and sorrow cannot be simultaneous, because the same thing in the same respect cannot be pleasing and displeasing. If, on the other hand, joy and sorrow, understood thus, be not of the same object in the same respect, but either of different objects, or of the same object in different respects, in that case joy and sorrow are not contrary to one another, so that nothing hinders a man from being joyful and sorrowful at the same time--for instance, if we see a good man suffer, we both rejoice at his goodness and at the same time grieve for his suffering. In this way a man may be displeased at having sinned, and be pleased at his displeasure together with his hope for pardon, so that his very sorrow is a matter of joy. Hence Augustine says [De vera et falsa Poenitentia, the authorship of which is unknown]: "The penitent should ever grieve and rejoice at his grief."
If, however, sorrow were altogether incompatible with joy, this would prevent the continuance, not of habitual penance, but only of actual penance.
Reply to Objection 3. According to the Philosopher (Ethic. ii, 3,6,7,9) it belongs to virtue to establish the mean in the passions. Now the sorrow which, in the sensitive appetite of the penitent, arises from the displeasure of his will, is a passion; wherefore it should be moderated according to virtue, and if it be excessive it is sinful, because it leads to despair, as the Apostle teaches (2 Corinthians 2:7), saying: "Lest such an one be swallowed up with overmuch sorrow." Accordingly comfort, of which the Apostle speaks, moderates sorrow but does not destroy it altogether.
Source: Summa Theologica (New Advent)