I think I (and others) frequently have an undeveloped understanding of sin. I tend to look at sin as individual acts only, as an aberration from who & what I really am. My sins appear to me as stand-alone enterprises, solitary acts that need to be repented of surely, but which are otherwise unconnected to me as a whole. I am beginning to see how wrong this view of sin actually is.
There is a strong tendency to assume that our sinful acts will ultimately be used for good. On the face of it, I agree, but the underlying assumption that is hardly ever voiced is that our sins do no damage to ourselves. We lust, get angry, lie or cheat, etc, and think that these actions are merely bobbing on the surface of our self. Once repented of (or perhaps before depending on our view of grace), these are plucked from the water and we are as we were before. But this is wrong - that underlying assumption is false. Certainly God can and does use our sins for our benefit - He works all things to good for those who love Him. We can learn from them, be strengthened by them, and through our own failings, we can help others to avoid those pitfalls, minister to them in their pain, build them up after they have fallen. However, our sins do not leave us undamaged. Each sin, especially the intentional ones, diminishes us. By our sins we are made the lesser. We are diminished in our impact, our faith, our ability to see & hear God, our ministry to others, our service to our King. I am not saying that God cannot heal us or that God cannot continue to use us - I am simply saying that each sin does not just empty the glass, it makes the glass smaller, just an injury can disable us. We subtract from our capacity to be used by God.