I wouldn't feel too badly, we all have done dumb things when we were younger, things we end up regretting. What's important is that you learned from it. That's a good thing. Many people never learn from their mistakes and just keep themselves stuck in the same destructive cycle.
I'm not sure how much time has passed, but most likely, your friends have moved on, as you have. That's normal too. I wouldn't worry about trying to rekindle friendships that have run their course. You certainly could contact your old friends (maybe via facebook, or other social media venue), and just offer a general apology, saying something like you said here, that you regret acting poorly, and feel badly about it. Then, just wish them well.
Again, that's up to you, you don't have to do that, but if it's weighing on your mind, it may make you feel better, and bring closure to the situation for you. You've grown and matured, which is what is important. You can use these life lessons to ensure you don't repeat that kind of behavior with your current friends.
I commend you for being a big enough person to admit your faults, and identify that you acted poorly, and owning it. That's a wonderful thing, it shows growth and maturity.
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