Is your ex my best friend from high school? The she’s definitely a lesbian now.
Just kidding.
I’d be asking this for real if you told me it happened a few years ago. Everything checks out. But I could be wrong. In either case, she probably needs to talk to someone. As in, a counselor or therapist. When she gets to college, that’ll be easy, but maybe she can talk to her parents about seeing someone until then. If she is questioning her sexuality, she might feel inclined to open up about it to a professional. And if not, then she should be finding out why she’s depressed.
As a friend, all you can do is let her know you still care. If you’re comfortable with it, tell her you’re concerned that she’s having problems she can’t handle herself right now and that there’s no shame in seeking help.
Things will work out in the end, and at the very least, you’re one of a small group of HS students who had a nice long relationship. Best of luck…