Baptism washes away all sins committed up to that point. Sins committed after baptism have to be repented of and confessed.
You are asking the wrong questions about the forgiveness of sins. Of course Catholics teach that Christ’s sacrifice totally merited our salvation. However, the question remains how his merits are applied to us. Now, except for an extreme minority of Protestants who believe contrary to te Scriptures that the elect are justified from the historical moment of Christ’s sacrifice, we all acknowledge that we are born in sin, and we are justified at some point in time in our lives. You would say that you were born a sinner, and were justified when you came to faith in Christ or something alon those lines, right? Does this mean that your faith what takes away the sins of the world? Not at all. The necessity of faith for salvation does not detract in the slightest from the sufficiency of Christ’s sacrifice. St. Paul says we are saved through faith.
The Council of Trent teaches,
Of this Justification the causes are these: the final cause indeed is the glory of God and of Jesus Christ, and life everlasting; while the efficient cause is a merciful God who washes and sanctifies gratuitously, signing, and anointing with the holy Spirit of promise, who is the pledge of our inheritance; but the meritorious cause is His most beloved only-begotten, our Lord Jesus Christ, who, when we were enemies, for the exceeding charity wherewith he loved us, merited Justification for us by His most holy Passion on the wood of the cross, and made satisfaction for us unto God the Father; the instrumental cause is the sacrament of baptism, which is the sacrament of faith, without which (faith) no man was ever justified; lastly, the alone formal cause is the justice of God, not that whereby He Himself is just, but that whereby He maketh us just, that, to wit, with which we being endowed by Him, are renewed in the spirit of our mind, and we are not only reputed, but are truly called, and are, just, receiving justice within us, each one according to his own measure, which the Holy Ghost distributes to every one as He wills, and according to each one’s proper disposition and co-operation. For, although no one can be just, but he to whom the merits of the Passion of our Lord Jesus Christ are communicated, yet is this done in the said justification of the impious, when by the merit of that same most holy Passion, the charity of God is poured forth, by the Holy Spirit, in the hearts of those that are justified, and is inherent therein: whence, man, through Jesus Christ, in whom he is ingrafted, receives, in the said justification, together with the remission of sins, all these (gifts) infused at once, faith, hope, and charity. For faith, unless hope and charity be added thereto, neither unites man perfectly with Christ, nor makes him a living member of His body. For which reason it is most truly said, that Faith without works is dead and profitless; and, In Christ Jesus neither circumcision, availeth anything, nor uncircumcision, but faith which worketh by charity.
history.hanover.edu/texts/trent/ct06.html
You would do well to read the whole Decree on Justification.