Objection 1. It would seem that a general confession does not suffice to blot out forgotten mortal sins. For there is no necessity to confess again a sin which has been blotted out by confession. If, therefore, forgotten sins were forgiven by a general confession, there would be no need to confess them when they are called to mind.
Objection 2. Further, whoever is not conscious of sin, either is not guilty of sin, or has forgotten his sin. If, therefore, mortal sins are forgiven by a general confession, whoever is not conscious of a mortal sin, can be certain that he is free from mortal sin, whenever he makes a general confession: which is contrary to what the Apostle says (1 Corinthians 4:4), "I am not conscious to myself of anything, yet am I not hereby justified."
Objection 3. Further, no man profits by neglect. Now a man cannot forget a mortal sin without neglect, before it is forgiven him. Therefore he does not profit by his forgetfulness so that the sin is forgiven him without special mention thereof in confession.
Objection 4. Further, that which the penitent knows nothing about is further from his knowledge than that which he has forgotten. Now a general confession does not blot out sins committed through ignorance, else heretics, who are not aware that certain things they have done are sinful, and certain simple people, would be absolved by a general confession, which is false. Therefore a general confession does not take away forgotten sins.
On the contrary, It is written (Psalm 33:6): "Come ye to Him and be enlightened, and your faces shall not be confounded." Now he who confesses all the sins of which he is conscious, approaches to God as much as he can: nor can more be required for him. Therefore he will not be confounded by being repelled, but will be forgiven.
Further, he that confesses is pardoned unless he be insincere. But he who confesses all the sins that he calls to mind, is not insincere through forgetting some, because he suffers from ignorance of fact, which excuses from sin. Therefore he receives forgiveness, and then the sins which he has forgotten, are loosened, since it is wicked to hope for half a pardon.
I answer that, Confession produces its effect, on the presupposition that there is contrition which blots out guilt: so that confession is directly ordained to the remission of punishment, which it causes in virtue of the shame which it includes, and by the power of the keys to which a man submits by confessing. Now it happens sometimes that by previous contrition a sin has been blotted out as to the guilt, either in a general way (if it was not remembered at the time) or in particular (and yet is forgotten before confession): and then general sacramental confession works for the remission of the punishment in virtue of the keys, to which man submits by confessing, provided he offers no obstacle so far as he is concerned: but so far as the shame of confessing a sin diminishes its punishment, the punishment for the sin for which a man does not express his shame, through failing to confess it to the priest, is not diminished.
Reply to Objection 1. In sacramental confession, not only is absolution required, but also the judgment of the priest who imposes satisfaction is awaited. Wherefore although the latter has given absolution, nevertheless the penitent is bound to confess in order to supply what was wanting to the sacramental confession.
Reply to Objection 2. As stated above, confession does not produce its effect, unless contrition be presupposed; concerning which no man can know whether it be true contrition, even as neither can one know for certain if he has grace. Consequently a man cannot know for certain whether a forgotten sin has been forgiven him in a general confession, although he may think so on account of certain conjectural signs.
Reply to Objection 3. He does not profit by his neglect, since he does not receive such full pardon, as he would otherwise have received, nor is his merit so great. Moreover he is bound to confess the sin when he calls it to mind.
Reply to Objection 4. Ignorance of the law does not excuse, because it is a sin by itself: but ignorance of fact does excuse. Therefore if a man omits to confess a sin, because he does not know it to be a sin, through ignorance of the Divine law, he is not excused from insincerity. on the other hand, he would be excused, if he did not know it to be a sin, through being unaware of some particular circumstance, for instance, if he had knowledge of another's wife, thinking her his own. Now forgetfulness of an act of sin comes under the head of ignorance of fact, wherefore it excuses from the sin of insincerity in confession, which is an obstacle to the fruit of absolution and confession.
Source: Summa Theologica (New Advent)