I think we get confused on this issue because our faith is not just about being cleansed from sin and then managing to avoid it for the rest of our lives. Sin and love are contradictory so that where one is decreasing the other must be increasing.
The CC teaches that justification is a process of conversion. She knows that anyone who lives past infancy will sin after Baptism-excepting Mary-but that nothing imperfect can enter heaven. God has a plan for us all and we’re either cooperating with it-which means we’re growing and being perfected in love-or we’re not. But He’s patient and longsuffering and “saves us while yet in our sins and transgressions” and He works in us in a myriad of ways, according to our unique personalities and life experiences, and He draws us, by His grace and without forcing us, to do those things which He’s set out for us to do. The rest is up to us.
**1989 The first work of the grace of the Holy Spirit is conversion, effecting justification in accordance with Jesus’ proclamation at the beginning of the Gospel: "Repent, for the kingdom of heaven is at hand."38 Moved by grace, man turns toward God and away from sin, thus accepting forgiveness and righteousness from on high. "Justification is not only the remission of sins, but also the sanctification and renewal of the interior man.39
1990 Justification detaches man from sin which contradicts the love of God, and purifies his heart of sin. Justification follows upon God’s merciful initiative of offering forgiveness. It reconciles man with God. It frees from the enslavement to sin, and it heals.
1993 Justification establishes cooperation between God’s grace and man’s freedom. On man’s part it is expressed by the assent of faith to the Word of God, which invites him to conversion, and in the cooperation of charity with the prompting of the Holy Spirit who precedes and preserves his assent:
When God touches man’s heart through the illumination of the Holy Spirit, man himself is not inactive while receiving that inspiration, since he could reject it; and yet, without God’s grace, he cannot by his own free will move himself toward justice in God’s sight.
2010 Since the initiative belongs to God in the order of grace, no one can merit the initial grace of forgiveness and justification, at the beginning of conversion. Moved by the Holy Spirit and by charity, we can then merit for ourselves and for others the graces needed for our sanctification, for the increase of grace and charity, and for the attainment of eternal life. Even temporal goods like health and friendship can be merited in accordance with God’s wisdom. These graces and goods are the object of Christian prayer. Prayer attends to the grace we need for meritorious actions.