A simple mistake, one that anyone could make, and isn't that true of all of them? Divorce, episodes of lousy parenting, dumb-head investments, negligence behind the wheel -- it's all too human, and you have to learn how to admit failure and walk away from it and not torment yourself. Sometimes the remorse is worse than the offense.
Google tells me that this is a Garrison Keillor quote. I don't read him, normally, so I was at a loss to understand where I could have come across it, but it looks like it was here, on Salon: http://www.salon.com/news/opinion/feature/2007/04/18/keillor/
At any rate, I remember where I was in my life when I read it and saved it -- I was at a place where I was able to forgive someone but he wasn't yet able to forgive himself.