Let me quote:
‘The declaration of the number of sins is another feature completing the Sacrament. The penitent must give the number of sins as far as he can; if he knows exactly how often he has fallen into a mortal sin, he must state that number of times, neither increasing nor diminishing; if, despite careful examination and reflection he cannot arrive at the real number, he must give it as near as possible, adding the words “about” or “at least”; in doing so he fulfills his obligation, for he has done what he could, which is sufficient to enable a judgement to be be pronounced humano modo. Should the penitent, after having confessed in all good faith, discover later on a more accurate number than that confessed, he is not obliged to make another confession to supply this number; nor should he disquiet himself, for the round numbers given in the first confession included everything; it is only when the newly discovered number is considerably greater than the vague estimate of his first confession that he is obliged to confess again, because the number, and in consequence, the sin, was not perfectly confessed, since a far greater number cannot be considered as included in his former round estimate.’
… ‘Indeed with habitual sinners it suffices to state how long they have indulged the evil habit, and that they have given willful consent more or less daily whenever the occasion offered; that is enough, when the actual number of sins is so doubtful that there wuld always be a grave risk of mistake in trying to determine it. “The confessor, when he knows the period over which the accusation extends, may easily and safely form his opinion in the case of the penitent whose will is habitually inclined to sin, that the penitent has sinned as often as there were necessary interruptions in his sin.”’
… 'Of course many distinguished theologians teach that whoever remembers a grave sin, even though not committed since the last confession but forgotten, must confess that sin and receive absolution before going to communion. The only reason urged is that he is conscious of this sin; and, according to the Council of Trent, no one who is conscious of a grave sin may receive communion before having confessed where there is an opportunity of making the confession.
It is permissible, however, with St. Alphonsus and other theologians (in less number) to follow the other “very probable opinion” which denies the obligation of confessing; for in reality the confession has preceded communion and the penitent has confessed all the sins of which he was conscious, so that neither the Council of Trent nor the divine law seems to demand more; moreover, the forgotten sin has been remitted indirectly, the penitent is in the state of grace, not merely by an act of contrition but in virtue of a valid confession. The practice of the faithful which is appealed to for the opposite side is not to be regarded as of binding force, but rather a pious and praiseworthy custom.’
- Theory and Practice of the Confessional, Prof. Caspar E. Schieler, D.D., Benziger Brothers, Imprimatur, Nihil Obstat [this is an instruction manual for priests]
In other words, the most important part of the Sacrament of Confession is the simple openness and honesty about the nature and number of your serious sins and the nature of your recollection of them.
I should also mention before Confession, but immediately after any sin, one should make an act of perfect contrition.
It is not hard for someone who tries to invalidate a Confession – one merely has to have a gravely sinful defect in one’s examination of conscience, deliberately conceal a grave sin, have no contrition and purpose of amendment, and there is more.
This is why people who approach Confession the wrong way with poor catechesis do not benefit.
And why Catholic knowledge and study is so important!
[edit – added a paragraph to the quote]
