Your question reflects the kind of thinking that says that “beauty is only skin deep, but ugly goes to the bone.”
Thinking one way, we are totally dependent upon grace, and only God’s grace helps us, sustains us, and saves us. A focus on our merits rather than on God’s love and grace is misplaced.
Another way it to look at the effects of grace on our lives: the slow growth of good habits (virtues) and the diminishment of bad habits (vices), with occasional setbacks, some serious, some less so. If you have worked hard to cooperate with God’s grace but stumble due to sin, even mortal sin, you don’t automatically revert to a person who has done nothing to turn toward God and try to live a godly life. After reconciliation with God, and relying on God’s grace, you resume the difficult task of conforming to God’s will and guidance. You don’t go back to square one: you don’t necessarily have to learn all over again to attend church, pray, serve others, frequent the sacraments, study. A setback is just that. You pick up where you left off.