The Act of Contrition is no substitute for sacramental Confession.
secondexodus.com/html/prayers/actofcontrition.htm
“For those who after baptism have fallen into sin, the Sacrament of Penance is as necessary unto salvation as is baptism itself for those who have not yet been regenerated” (Council of Trent, Sess. XIV, c. 2)
An act of contrition suffices to obtain forgiveness of venial sin, and the same effect is produced by the worthy reception of sacraments other than penance.
newadvent.org/cathen/11618c.htm
Of course we know that Jesus is not limited to forgive by the sacraments, and there are plenary indulgences. God bless.
In my explanation I made sure to note that I mentioned the Act must be made when in danger of death and recourse to a Priest is impossible. Only when these conditions are met would the Act be able to count as a Sacramental Confession. Furthermore this allowance comes with the caveat that if the person lives he must still confess those sins because without the imminent threat of death and realization of that threat the Act does not count as a Sacramental Confession. And what the heck even though I’m at work I will copy and paste this excerpt from the New Advent’s online Catholic Encyclopedia:
Perfect contrition without the sacrament
Regarding that contrition which has for its motive the love of God, the Council of Trent declares: “The Council further teaches that, though contrition may sometimes be made perfect by charity and may reconcile men to God before the actual reception of this sacrament, still the reconciliation is not to be ascribed to the contrition apart from the desire for the sacrament which it includes.” The following proposition (no. 32) taken from Baius was condemned by Gregory XIII: “That charity which is the fullness of the law is not always conjoined with forgiveness of sins.” Perfect contrition, with the desire of receiving the Sacrament of Penance, restores the sinner to grace at once. This is certainly the teaching of the Scholastic doctors (Peter Lombard in P.L., CXCII, 885; St. Thomas, In Lib. Sent. IV, ibid.; St. Bonaventure, In Lib. Sent. IV, ibid.). This doctrine they derived from Holy Writ. Scripture certainly ascribes to charity and the love of God the power to take away sin: “He that loveth me shall be loved by My Father”; “Many sins are forgiven her because she hath loved much”. Since the act of perfect contrition implies necessarily this same love of God, theologians have ascribed to perfect contrition what Scripture teaches belongs to charity. Nor is this strange, for in the Old Covenant there was some way of recovering God’s grace once man had sinned. God wills not the death of the wicked, but that the wicked turn from his way and live (Ezekiel 33:11). This total turning to God corresponds to our idea of perfect contrition; and if under the Old Law love sufficed for the pardon of the sinner, surely the coming of Christ and the institution of the Sacrament of Penance cannot be supposed to have increased the difficulty of obtaining forgiveness. That the earlier Fathers taught the efficacy of sorrow for the remission of sins is very clear (Clement in P.G., I, 341 sqq.; and Hermas in P.G., II, 894 sqq.; Chrysostom in P.G., XLIX, 285 sqq.) and this is particularly noticeable in all the commentaries on Luke 7:47. The Venerable Bede writes (P.L., XCII, 425): “What is love but fire; what is sin but rust? Hence it is said, many sins are forgiven her because she hat loved much, as though to say, she hath burned away entirely the rust of sin, because she is inflamed with the fire of love.” Theologians have inquired with much learning as to the kind of love that justifies with the Sacrament of Penance. All are agreed that pure, or disinterested, love (amor benevolentiæ, amor amicitiæ) suffices; when there is question of interested, or selfish, love (amor concupiscentia) theologians hold that purely selfish love is not sufficient. When on furthermore asks what must be the formal motive in perfect love, there seems to be no real unanimity among the doctors. Some say that where there is perfect love God is loved for His great goodness alone; other, basing their contention on Scripture, think that the love of gratitude (amor gratitudinis) is quite sufficient, because God’s benevolence and love towards men are intimately united, nay, inseparable from His Divine perfections (Hurter, Theol. Dog., Thesis ccxlv, Scholion iii, no 3; Schieler-Heuser, op. cit., pp. 77 sq.).