In the long run I don't really regret (Or at least I try not too). Sometimes there are things that I said and I wish I could take back, but what can you do? Learn from it, pick yourself up, and move on.
However, my biggest regret is probably not taking Algebra I in seventh grade. I instead took Pre-Algebra, even though I was recommended for Algebra I. I took the easy way out and now I'm in honors-average math instead of high-honors math. I think it denied me some opportunites that I might otherwise have had.
But I learned from it and now I know not to sell myself short and to take advantage of every opportunity. It also taught me to believe in myself and know that I am capable of what I may not realize.