We all make mistakes. We're only human. What is important is how we learn from those mistakes.
In terms of myself and my personal development I could regret a lot of things. I could regret not going to uni, not learning how to drive at 16, not having moved into my own place properly. etc etc
But for each of these things that I could regret there are good things - or at least reasons - that have come out of them.
I think the only regrets I can truly say I have is for the way I have treated some people. Unfairly, often harshly and more often than not because I was being extremely selfish. Very often in affairs of the heart. I could say I was young and stupid and whilst that may hold some sway, on the whole I'm an intelligent girl and I should've known better.
I often think there's no changing the past. Maybe there is. Maybe the boy who's heart I no doubt broke because I went after something "better" (read as: More attainable, he wasnt half way across the country) could feel a sense of peace if I found his address in an old diary and wrote to him to apologise. Maybe you should let sleeping dogs lie? Who knows.
All I know is that the things I have done wrong...and for the things I will no doubt continue to do wrong (hey - I am only human!) I hope I will learn from them, move forward and try and prevent myself having those same regrets again.
And then the only thing I could ever truly regret is not having learnt from my mistakes.