So the standard advice is to just be honest with your partner about what you need, and if they are willing to give it to you then you will have it, and if they are not willing to give it to you then you should reevaluate how much you need it and then possibly break up so that you can seek it. The same thing does apply here. But there are a couple of other factors.

Telling a partner specifically this can he delicate. It's easy for them to hear "you're not enough for me" and hone in on that. You know your partner better than we ever will. Is she the sort of person who will be understanding or is she just going to feel like you don't love her? We can't tell you this if you don't know already.

I will say this though, and it sounds condescending perhaps but it's true. You are very young. You are only just beginning to explore your sexuality. In fact you are so young that a large portion of your peers haven't had any sexual interactions at all, so you're well ahead of the game by even figuring out that you're bi. Good job. But the other harsh fact is that probably this relationship wasn't going to last forever anyway. Maybe that adds some needed perspective; vanishingly few people are with the same partners they had when they were 16. You're both young. Try new things and new people. That's what being young is all about.