Objection 1. It would seem that it is not necessary to have contrition for each mortal sin. For the movement of contrition in justification is instantaneous: whereas a man cannot think of every mortal sin in an instant. Therefore it is not necessary to have contrition for each mortal sin.
Objection 2. Further, contrition should be for sins, inasmuch as they turn us away from God, because we need not be contrite for turning to creatures without turning away from God. Now all mortal sins agree in turning us away from God. Therefore one contrition for all is sufficient.
Objection 3. Further, mortal sins have more in common with one another, than actual and original sin. Now one Baptism blots out all sins both actual and original. Therefore one general contrition blots out all mortal sins.
On the contrary, For diverse diseases there are diverse remedies, since "what heals the eye will not heal the foot," as Jerome says (Super Marc. ix, 28). But contrition is the special remedy for one mortal sin. Therefore one general contrition for all mortal sins does not suffice.
Further, contrition is expressed by confession. But it is necessary to confess each mortal sin. Therefore it is necessary to have contrition for each mortal sin.
I answer that, Contrition may be considered in two ways, as to its origin, and as to its term. By origin of contrition I mean the process of thought, when a man thinks of his sin and is sorry for it, albeit not with the sorrow of contrition, yet with that of attrition. The term of contrition is when that sorrow is already quickened by grace. Accordingly, as regards the origin of contrition, a man needs to be contrite for each sin that he calls to mind; but as regards its term, it suffices for him to have one general contrition for all, because then the movement of his contrition acts in virtue of all his preceding dispositions.
This suffices for the Reply to the First Objection.
Reply to Objection 2. Although all mortal sins agree in turning man away from God, yet they differ in the cause and mode of aversion, and in the degree of separation from God; and this regards the different ways in which they turn us to creatures.
Reply to Objection 3. Baptism acts in virtue of Christ's merit, Who had infinite power for the blotting out of all sins; and so for all sins one Baptism suffices. But in contrition, in addition to the merit of Christ, an act of ours is requisite, which must, therefore, correspond to each sin, since it has not infinite power for contrition.
It may also be replied that Baptism is a spiritual generation; whereas Penance, as regards contrition and its other parts, is a kind of spiritual healing by way of some alteration. Now it is evident in the generation of a body, accompanied by corruption of another body, that all the accidents contrary to the thing generated, and which were the accidents of the thing corrupted, are removed by the one generation: whereas in alteration, only that accident is removed which was contrary to the accident which is the term of the alteration. In like manner, one Baptism blots out all sins together and introduces a new life; whereas Penance does not blot out each sin, unless it be directed to each. For this reason it is necessary to be contrite for, and to confess each sin.
Source: Summa Theologica (New Advent)