Maybe I replied too quickly. Do you mean you and she went out a couple of times but you gave up after you found out she went out with some more popular boy, too?
And to answer the question here. I have tons of things I'd do differently, but I don't have any painful regrets.
1. As others have said, I would have asked more girls out. My next door neighbor had a huge crush on me. She was all kinds of hot.
2. Studied harder. Or at all.
3. Trained harder. I was not a very good runner in HS. This was mostly because I didn't train very hard - well, that and a lack of talent..