From the institution of the Sacrament of Penance as already explained, the Universal Church has always understood, that the entire confession of sins was also instituted by the Lord, and is of divine right necessary for all who have fallen after Baptism; because that our Lord Jesus Christ, when about to ascend from earth to heaven, left priests His own vicars, as presidents and judges, unto whom all the mortal crimes, into which the faithful of Christ may have fallen, should be carried, in order that, in accordance with the power of the keys, they may pronounce the sentence of forgiveness or retention of sins. For it is manifest, that priests could not have exercised this judgment without knowledge of the cause; neither indeed could they have observed equity in enjoining punishments, if the said faithful should have declared their sins in general only, and not rather specifically, and one by one. Whence it is gathered that all the mortal sins, of which, after a diligent examination of themselves, they are conscious, must needs be by penitents enumerated in confession, even though those sins be most hidden, and committed only against the two last precepts of the Decalogue, - sins which sometimes wound the soul more grievously, and are more dangerous, than those which are committed outwardly. For venial sins, whereby we are not excluded from the grace of God, and into which we fall more frequently, although they be rightly and profitably, and without any presumption declared in Confession, as the custom of pious persons demonstrates, yet may they be omitted without guilt, and be expiated by many other remedies. But, whereas all mortal sins, even those of thought, render men children of wrath, and enemies of God, it is necessary to seek also for the pardon of them all from God, with an open and modest confession. Wherefore, while the faithful of Christ are careful to confess all the sins which occur to their memory, they without doubt lay them all bare before the mercy of God to be pardoned: whereas they who act otherwise, and knowingly keep back certain sins, such set nothing before the divine bounty to be forgiven through the priest: for if the sick be ashamed to show his wound to the physician, his medical art cures not that which it knows not of. We gather furthermore, that those circumstances which change the species of the sin are also to be explained in confession, because that, without them, the sins themselves are neither entirely set forth by the penitents, nor are they known clearly to the judges; and it cannot be that they can estimate rightly the grievousness of the crimes, and impose on the penitents, the punishment which ought to be inflicted, on account of them. Whence it is unreasonable to teach, that these circumstances have been invented by idle men; or, that one circumstance only is to be confessed, to wit, that one has sinned against a brother. But it is also impious to assert, that confession, enjoined to be made in this manner, is impossible, or to call it a slaughter-house of consciences: for it is certain, that in the Church nothing else is required of penitents, but that, after each has examined himself diligently, and searched all the folds and recesses of his conscience, he confess those sins by which he shall remember that he has mortally offended his Lord and God: whilst the other sins, which do not occur to him after diligent thought, are understood to be included as a whole in that same confession; for which sins we confidently say with the prophet; “From my secret sins cleanse me, O Lord.” Now, the very difficulty of a confession like this, and the shame of making known one’s sins, might indeed seem a grievous thing, were it not alleviated by the so many and so great advantages and consolations, which are most assuredly bestowed by absolution upon all who worthily approach to this sacrament… Wherefore, whereas the secret sacramental confession, which was in use from the beginning in Holy Church, and is still also in use, has always been commended by the most holy and the most ancient Fathers with a great and unanimous consent, the vain calumny of those is manifestly refuted, who are not ashamed to teach, that confession is alien from the divine command, and is a human invention, and that it took its rise from the Fathers assembled in the Council of Lateran: for the Church did not, through the Council of Lateran, ordain that the faithful of Christ should confess, - a thing which it knew to be necessary, and to be instituted of divine right, - but that the precept of Confession should be complied with, at least once a year, by all and each, when they have attained to years of discretion. Whence, throughout the whole Church, the salutary custom is, to the great benefit of the souls of the faithful, now observed, of confessing at that most sacred and most acceptable time of Lent, - a custom which this Holy Synod most highly approves of and embraces, as pious and worthy of being retained.
In other words, you must confess all mortal sins (in number and species) that you have committed after Baptism. This is bound upon you under the assent of faith. You cannot receive Holy Communion until this is done (with the other parts of Penance), as is said above, if you knowingly withhold certain sins, you set NOTHING before the divine bounty to be forgiven (i.e., you’re still in a state of mortal sin). Even while Trent states, “the other sins, which do not occur to him after diligent thought, are understood to be included as a whole in that same confession,” that doesn’t remove the necessity of the confession of those mortal sins in species and number once one DOES remember them.