Now repentance is no fun at all. It is something much harder than merely eating humble pie. It means unlearning all the self-conceit and self-will that we have been training ourselves into for thousands of years. It means killing part of yourself, undergoing a kind of death.
In fact, it needs a good man to repent. And here comes the catch. Only a bad person needs to repent: only a good person can repent perfectly. The worse you are the more you need it and the less you can do it. The only person who could do it perfectly would be a perfect person – and he would not need it.
Remember, this repentance, this willing submission to humiliation and a kind of death, is not something God demands of you before He will take you back and which He could let you off if He chose: it is simply a description of what going back to Him is like. If you ask God to take you back without it, you are really asking Him to let you go back without going back. It cannot happen…
But the same badness which makes us need it, makes us unable to do it. Can we do it if God helps us? Yes, but what do we mean when we talk of God helping us? We mean God putting into us a bit of Himself so to speak. He lends us a little of His reasoning powers and that is how we think: He puts a little of His love into us and that is how we love on another. 
The above paragraphs come from CS Lewis’ Mere Christianity. It is a rather humbling experience when one comes to the point of realizing that he cannot repent, which as stated above, means far more than saying “I’m sorry.” It comes from the Greek word, metanoia, which literally means “to do an about face, to turn around.” In other words, repentance means to completely orient one’s being away from the sin that has been committed.
Deep, true repentance is impossible on our own which is why it requires a kind of death. But we have the God-man Jesus Christ who is willing to work from within us, directing us on this path of repentance. When we allow our ineptitude to humble us before the feet of Christ, we then begin to find something much stronger working within.
 Mere Christianity by CS Lewis, page 54-55, from the chapter entitled The Perfect Penitent. Harper One, 2002.