Regret seems pretty pointless in the end: if it's happened (or not happened) and there's nothing you can do about it, all it can do is make you feel bad. So I try to learn from the experience, then accept it and move on.
(I'm trying really hard to avoid feelings that don't have any upside these days. It's the same with fear: a little keeps you sharp and stops you making mistakes or bad decisions, but past that it's not helpful. Seems to be harder to get rid of, though. :/)
Accept, everyone deserves my acceptance