Guilt.

First do not harm.
Easy said than done.
But the fact is most of us do harm all the time.
Sometimes even when we're trying to help,
we do more harm than good
and then the guilt rolls in our head.
What you do with that guilt is up to you.
We're left with a choice.
Either let the guilt throw u back into
the hero that got you into trouble in the first place
or learn from the guilt and do your best to move on.

extracted from grey's anatomy, season 3