I think talking to a psych might be useful. Even if you decide not to treat anything right now, having some professional input could be really helpful. You might think about talking to your school counselor or psychologist as well. They can let you know about resources for teens in your area.
Muster up the courage and talk to your friend tomorrow. If it helps, think about a few topics you can discuss in advance, so if there are any pauses in the conversation you'll have some ideas about how to fill them in. That will help you feel more confident. Remember, this person is your friend -- even if she is talking with someone else too. She enjoys your company. Let her know you'd still like to spend time with her. Propose a movie night or something else low-key. She may be feeling uncertain about your relationship too, but that doesn't mean she doesn't like you anymore.
I hope this is helpful. You can do it!
Co-Moderator, Bipolar Forum
"Bipolar disorder can be a great teacher. It's a challenge, but it can set you up to be able to do almost anything else in your life." - CARRIE FISHER