I'm not gonna lie my dude this just sounds like average horny teenage behavior. You're not the first one that urges have taken control of and you won't be the last. It's okay to have regret about it, but it was a consensual thing and many of us have experienced similar things when we're young, or sometimes even after growing up (hopefully not because that shit doesn't work in the adult world.). We call these "flings".
Neither of you really knew what you were doing, by the sounds of it, if you were as young as it sounds, you were likely just feeling good with someone and wanted to chase the feeling and probably didn't know how to make a functional relationship work at that age (most everyone doesn't. We just always pretended we did.). It's a very common thing, and it's also common for people, friends, flames and partners to come and go out of our lives. We grow, we learn and that process never stops.
She was your first, as they say you always remember your first. I definitely remember mine and often used to wonder what ifs myself, but I don't really think much about her anymore after she used to haunt me like a ghost. Obviously this is a memory and thoughts that mean something to you, so saying "don't sweat it" will probably mean very little, but...
Don't sweat it. You had a perfectly normal experience.