Not a lot, because what's happened to us, good or bad, makes us what we are now. I decided regrets and guilt were wasted time, a long time ago. There are a couple of things I've had a harder time getting rid of, though.
1) When I was in high school, there was a guy in my debate class that just adored me. I wasn't all that and a big of chips, so it should have been flattering. Instead, one day when he came and sat next to me, my mouth got away with me. I huffed loudly and said, "Oh, crap!", to which my buddy replied, "Let's move." There weren't a lot of people around, but there were definitely people who heard the whole thing. Cruelest thing I've ever done to anyone, and if I could go back and change that one thing, I would.
2) The night before my dad died, I was at my parents' house with the kids, chatting. When it was time to leave, my dad was out on the deck, smoking. I was busy bundling up kids (it was Feb), so instead of going out to tell him goodnight and give him a hug, I yelled, "Bye, Dad!" at the back of his head. If I could only change one thing, I'd change that.