Thomas Aquinas, Compendium of Theology

This enables us to perceive that mortal sins are not forgiven in the next world. But venial sins are forgiven. Mortal sins are committed by turning away from our last end, in which man is irrevocably settled after death, as we have just said. Venial sins, however, do not regard our last end, but rather the road leading to that end.

If the will of evil men is obstinately fettered to evil after death, they forever continue to desire what they previously desired, in the conviction that this is the best. Therefore they are not sorry they have sinned; for no one is sorry he has achieved what he judges to be the best.

But we should understand that those who are condemned to final misery cannot have after death what they craved as the best. Libertines in hell will have no opportunity to gratify their passions; the wrathful and the envious will have no victims to offend or obstruct; and so of all the vices in turn. But the condemned will be aware that men who have lived a virtuous life in conformity with the precepts of virtue obtain what they desired as best. Therefore the wicked regret the sins they have committed, not because sin displeases them, for even in hell they would rather commit those same sins, if they had the chance, than possess God; but because they cannot have what they have chosen, and can have only what they have detested. Hence their will must remain forever obstinate in evil, and at the same time they will grieve most agonizingly for the sins they have committed and the glory they have lost. This anguish is called remorse of conscience, and in Scripture is referred to metaphorically as a worm, as we read in Isaias 66:24: "Their worm shall not die."