This is my understanding but it may be incorrect.
If we pray a private act of contrition, God will forgive our sins but that doesn’t mean we have been absolved (made the situation right with God) or are still not in communion with his Church and can’t recieve the sacraments from which we get and need other graces to perservere and succeed spiritually in this world. But an Act of Contrition can result in absolution if the Sacrament of Penance is not available and does not become available before our death (i.e. We fall from a cliff and say the Act or say as much as we can before we hit the bottom. This is why my Grandma also said Oh my God whenever she slipped or I almost rolled the car learning to drive on the rural gravel roads near our farm.)
CCC: 1446
Christ instituted the sacrament of Penance for all sinful members of his Church: above all for those who, since Baptism, have fallen into grave sin, and have thus lost their baptismal grace and wounded ecclesial communion. It is to them that the sacrament of Penance offers a new possibility to convert and to recover the grace of justification. The Fathers of the Church present this sacrament as “the second plank [of salvation] after the shipwreck which is the loss of grace.”
Additionally, it is not just God’s forgiveness that we need for salvation (at least without a period of purging and cleansing in purgatory) but we must also pledge to reform our lives or else we will fall back into the sin that offends our God.
CCC 1428
Christ’s call to conversion continues to resound in the lives of Christians. This second conversion is an uninterrupted task for the whole Church who, "clasping sinners to her bosom, [is] at once holy and always in need of purification, [and] follows constantly the path of penance and renewal."18 This endeavor of conversion is not just a human work. It is the movement of a “contrite heart,” drawn and moved by grace to respond to the merciful love of God who loved us first
And we must have contrition.
CCC: 1453
The contrition called “imperfect” (or “attrition”) is also a gift of God, a prompting of the Holy Spirit. It is born of the consideration of sin’s ugliness or the fear of eternal damnation and the other penalties threatening the sinner (contrition of fear). Such a stirring of conscience can initiate an interior process which, under the prompting of grace, will be brought to completion by sacramental absolution. By itself however, imperfect contrition cannot obtain the forgiveness of grave sins, but it disposes one to obtain forgiveness in the sacrament of Penance.
And finally, we must perform penance.
CCC: 1459
Many sins wrong our neighbor. One must do what is possible in order to repair the harm (e.g., return stolen goods, restore the reputation of someone slandered, pay compensation for injuries). Simple justice requires as much. But sin also injures and weakens the sinner himself, as well as his relationships with God and neighbor. Absolution takes away sin, but it does not remedy all the disorders sin has caused.62 Raised up from sin, the sinner must still recover his full spiritual health by doing something more to make amends for the sin: he must “make satisfaction for” or “expiate” his sins. This satisfaction is also called “penance.”
