Imagine an artist's canvas. Blank, clean and pure. Then the painter puts his brush to it. He can never guarantee that it won't go wrong, yet he puts full faith into his skills and takes a leap.
Now, let's just say he does make a mistake, a huge dark smudge over the whole picture, tainting it, contaminating it. It is never going to be the same. Yes, he can paint over it flawlessly; no one else will know that it is there. But he always will, he could stand receiving even the highest form of praise and yet have gritted teeth. All because of that tugging sensation, reminding him that it is not perfect. Underneath the thick brush strokes, there is an ever present black stain, deforming it from the inside out.
Okay, now think about the same thing, only this time the blank canvas is a person. It could be you or me, anyone. To me, it is what it is like when you've done something you know you shouldn't have. When you know you've hurt someone. How ever efficiently you hide it from other people, you will always know.