Objection 1. It would seem that sorrow for one sin need not be greater than for another. For Jerome (Ep. cviii) commends Paula for that "she deplored her slightest sins as much as great ones." Therefore one need not be more sorry for one sin than for another.
Objection 2. Further, the movement of contrition is instantaneous. Now one instantaneous movement cannot be at the same time more intense and more remiss. Therefore contrition for one sin need not be greater than for another.
Objection 3. Further, contrition is for sin chiefly as turning us away from God. But all mortal sins agree in turning us away from God, since they all deprive us of grace whereby the soul is united to God. Therefore we should have equal contrition for all mortal sins.
On the contrary, It is written (Deuteronomy 25:2): "According to the measure of the sin, shall the measure also of the stripes be." Now, in contrition, the stripes are measured according to the sins, because to contrition is united the purpose of making satisfaction. Therefore contrition should be for one sin more than for another.
Further, man should be contrite for that which he ought to have avoided. But he ought to avoid one sin more than another, if that sin is more grievous, and it be necessary to do one or the other. Therefore, in like manner, he ought to be more sorry for one, viz. the more grievous, than for the other.
I answer that, We may speak of contrition in two ways: first, in so far as it corresponds to each single sin, and thus, as regards the sorrow in the higher appetite, a man ought to be more sorry for a more grievous sin, because there is more reason for sorrow, viz. the offense against God, in such a sin than in another, since the more inordinate the act is, the more it offends God. In like manner, since the greater sin deserves a greater punishment, the sorrow also of the sensitive part, in so far as it is voluntarily undergone for sin, as the punishment thereof, ought to be greater where the sin is greater. But in so far as the emotions of the lower appetite result from the impression of the higher appetite, the degree of sorrow depends on the disposition of the lower faculty to the reception of impressions from the higher faculty, and not on the greatness of the sin.
Secondly, contrition may be taken in so far as it is directed to all one's sins together, as in the act of justification. Such contrition arises either from the consideration of each single sin, and thus although it is but one act, yet the distinction of the sins remains virtually therein; or, at least, it includes the purpose of thinking of each sin; and in this way too it is habitually more for one than for another.
Reply to Objection 1. Paula is commended, not for deploring all her sins equally, but because she grieved for her slight sins as much as though they were grave sins, in comparison with other persons who grieve for their sins: but for graver sins she would have grieved much more.
Reply to Objection 2. In that instantaneous movement of contrition, although it is not possible to find an actually distinct intensity in respect of each individual sin, yet it is found in the way explained above; and also in another way, in so far as, in this general contrition, each individual sin is related to that particular motive of sorrow which occurs to the contrite person, viz. the offense against God. For he who loves a whole, loves its parts potentially although not actually, and accordingly he loves some parts more and some less, in proportion to their relation to the whole; thus he who loves a community, virtually loves each one more or less according to their respective relations to the common good. In like manner he who is sorry for having offended God, implicitly grieves for his different sins in different ways, according as by them he offended God more or less.
Reply to Objection 3. Although each mortal sin turns us away from God and deprives us of His grace, yet some remove us further away than others, inasmuch as through their inordinateness they become more out of harmony with the order of the Divine goodness, than others do.
Source: Summa Theologica (New Advent)