It’s a matter of the will. We don’t really have a “sinful nature”; that’s more of a Protestant idea. We have a will which is not entirely oriented and committed to God, an orientation that happens to be our ultimate good. Adam questioned whether or not God was worth heeding, whether or not God really was God IOW, higher than himself, to put it simply. Whether or not Adam might not be able to fill God’s shoes by himself. We’re here to learn that Adam was wrong. We’re here to gain the “knowledge of God” which Jesus came to reveal when the time was ripe, to reveal in no uncertain terms, as God had never been revealed before. Man is made for communion with God and is lost, dead, unable to rise, unable to overcome sin without Him.
"Apart from Me you can do nothing." John 15:5.
Separation from God is the heart of man’s problems, which explains why that very separation is the chief aspect of the state known as “original sin”. With the help of lived-experience in this relatively god-free world where both good and evil are literally known, combined with revelation and grace, we may come to see our need for God, and be able to recognize God in Jesus-in His words and deeds, and so of God’s existence, trustworthiness, mercy, goodness, and extraordinary love for humankind. Then our wills begin to change. Then we can be ready to receive the light, to be reconciled with God and enter communion with Him as is the right order of things for man; we can be justified, IOW.
"Now this is eternal life: that they know you, the only true God, and Jesus Christ, whom you have sent." John 17:3
From the catechism:
1731 Freedom is the power, rooted in reason and will, to act or not to act, to do this or that, and so to perform deliberate actions on one’s own responsibility. By free will one shapes one’s own life. Human freedom is a force for growth and maturity in truth and goodness; it attains its perfection when directed toward God, our beatitude.
1732 As long as freedom has not bound itself definitively to its ultimate good which is God, there is the possibility of choosing between good and evil , and thus of growing in perfection or of failing and sinning. This freedom characterizes properly human acts. It is the basis of praise or blame, merit or reproach.
The purpose of purgatory is the final education and formation of the will, as we’re purified of anything that might distract it from it’s ultimate good, which is God. Then we’re free to fully love Him as we should, with our whole heart, soul, mind, and strength.