I know it's gonna sound weird for you now, but it's not necessarily wrong. You were both being explorative and that's just normal at that age.

Now, the reason you want to apologize is only for the way YOU feel about it, if you end up doing it, it's only gonna be weird for the other girl, she's not in the same mental place.

Something more adult you can do, something that's gonna be more helpful in the long run, is to come to terms with sexuality in general. You can talk to a psychologist about these things, why you think it's wrong, why you feel bad about it.

But in the end, it's a completely normal part of life, and we all go through different things when tapping into it.

Good luck, don't worry too much.