Christ, that's a difficult question to answer.

My knee jerks toward just saying "None - whatever my problems I like who I am and changing the past would also change who I am now. And I don't believe in changing history, only the future."

More accurately, I don't see any profit in regretting what did happen in the past - but in my not-so-secret heart of hearts I do sometimes regret things that didn't.

Still, I don't think I would erase any of them. I can't blame my lack of productivity on my relationships (romantic or otherwise). Even when in the passionate throes of first love I could have found some time work on that novel so I am, again, left with only myself to blame.