What do you consider your life's biggest mistake? Is it reversible and/or does it really need to be reversed? On what scale can we rate negative things that we've caused to ourselves, since it is a bit tricky to escape bias for our self-impression...
Is there anyone out there who hasn't ever made a mistake, a bad judgement call, something they wish they could take back? What a boring life you must lead, whoever you are!
I think those invisible scars, the things we think back upon and regret (with a hint of nostalgia?) are the best hints at who we really are, how well you know whoever is inside your body. Perhaps I say this as someone who has never murdered someone or an equivalent ultimate action, but I take comfort in my messups and wear them around my shoulders like a nice (faux) fur coat. Recognizing them makes me own me/know me, even if I happen to commit those same mistakes again.
[This isn't meant to be such a dreadful/depressing little blip on your blog radar, just celebrating imperfection... a wonderful thing!]