I feel like the mistakes I've made so far have been a necessary part of the learning process. Even if I could change one momentary decision, my attitude would've still been the same and led down a similar path. For example, there's a girl I liked in college but I was awkward about it and didn't ask her out. I think even if I had done so however the relationship wouldn't have been so great because I was still dealing with my own insecurities. Other stuff I found out along the way through meeting people at the right moment, like finally realizing how messed up my family was, but again I don't think knowing that earlier on would've made a difference until I was in a more receptive state.
if i can think of one thing though...hmm...perhaps pick a major outside my comfort zone in college, like computer science, and read what i was actually interested in on the side as a kind of hobby, since i was motivated to do it either way. then again my grades would've probably sucked.
either that or spend more time learning languages. some kind of difficult specialized knowledge that's much easier to acquire when all i had to do was take classes, rather than when i'm now working or doing research.