This is a distinction we desperately need to get clear. There are many Christians who are convinced – based on their despair, regret, and self-loathing – that they are repentant. But in reality, they are not repenting at all. Jack Miller helps to elucidate the difference in his excellent little book on Repentance:

Penance… is a religious attitude deeply rooted in the human heart which prompts people to attempt to pay for their own sins by good works and sufferings. Self-justification is the goal of this effort. In practice this means that humanity always has one more scheme for getting things right with God and their conscience. Sinners doing penance always say in their hearts, “Give me one more day, a new religious duty, another program, another set of human relationships or a better education, and then things will come right-side up.

They are preparationists – that is, sinners who are forever getting ready for grace. They want to make themselves worthy of grace so that God will reach out to them once this work of preparation is completed… But they do not know that this is a terrible insult to God and His grace. In their pride they are attempting the impossible… if we are grafted into Christ, if we are rooted in Christ, then we can grow in grace. But we will never have the power to grow into grace as a work of moral reformation.

Therefore, anyone doing penance is sadly mistaken. Things cannot come right for such people. They cannot pay for their sins, because they poison all the best gifts of God. Send them to church and Christian schools for a lifetime and they will never come to know rightly a single thing about the living God and his mercy in Christ. For in their heart of hearts, they are proud – infinitely proud – perhaps without having the slightest idea that this is their basic problem. Having but themselves as the ground of their hope, they will not see the glory of Christ until the Spirit grants them “repentance to life,” which included a genuine turning from penance. In brief, they must repent of their penances.

This matter is very tricky. Self-deception goes right along with self-trust and self-justification (Jer 17:9). You may say, “But you don’t know how earnestly I pray for God’s help. I have shed many tears over my sins.” But friend, this cannot work, because at bottom you are still asking God to baptize your sin – to Christianize an essentially lustful heart by making you a little less nervous and a little more patient. The Heavenly Father, however, does not hear your prayers, because you are in reality asking Him for help so that you can continue to live a life which is independent of God.

What these people seek from God is enough grace to be strong in themselves. They do not need or want a constant flow of water from heaven.

…The repentant person repudiates this whole process with its self-justification and pretense. For truly repentant sinners have discovered, through the renewing work of the Holy Spirit, that all their doing is full of sin. Their doing is the source of their wretched emptiness, their black depression and their self-despising. But now they have come undone. They turn from their sinful doing and trust in what Christ has done. This is the essence of repentance.

– from C. John Miller, Repentance (Fort Washington, PA: CLC Publications, 2010), 17-20.