I probably would have tried a lot harder to make sure my emotional health didn't go as south as it did. Although that happened even with me knowing that things are never as bad as I'm making them out to be and they will get better, so I'm not sure how much that would have helped.
Also, I would have tried harder to cultivate certain things and talents. I can still do that now, but it would be easier if I'd also done that back then instead of pretty much giving up on things. :/