You ask what people in their 30s, 40s, and older regret when they look back at their lives. I suppose it’s wise of you as a teenager to think about how you might avoid a future regret. But I’d counsel you to take any good path open to you, do the best you can, and be happy. Save regrets for your 50s and sixties. And you will have them. This may sound a little melodramatic, but no matter how happy you are, at my age, your regrets are countless. You have made decades' worth of little miscalculations you can't completely erase from your memory, as well as a number of big mistakes that made life permanently harder.
Divide regrets into three groups:
The things you did that you wish you hadn't.
At your young age, you most likely exaggerate the impact of your previous errors. If you've done something truly and dreadfully wrong, you are the reason we have a system that protects the identity of juveniles. Either way, you have a lot of time to recover.
By age 50, you laugh (if uneasily) about the mistakes you made when you were younger. After all, you see young people making those same mistakes, even when you have drawn them a picture of how to avoid them. Your sins of commission somehow seem not so disturbing, because in all but the worst cases, they're part of your story now, even if they left scars. You can joke about them with your oldest friends and your family. They played a part in making you who you are. Forgive yourself.