How to move on, on a broken friendship?

[Respect]

Well Iam 16 yrs old guy. I have a bestfriend (well used to). I know I made some mistakes in the past but when I tried to talk to her, She just walked-out then when I ask why? she doesn't speak at all.The following day I discovered that she has a "something" with this other guy which Iam way better than at every way even when it comes to a way thinking. I just gave her a SPACE for a month and this is what i discovered. and we've known each other for 4 yrs!. and the next thing happen they make conversation in twitter instead doing it privately, I never knew that she could be Inmature. I like her of course because she is so mature and when she speaks it all make sense but now She is really changed. I really dislike her now but of course she is still my friend I want to do things that is for the best and hoping we could find time to talk to each other and clear things up before we go to college and meet new people and bring back our friendship. One of my problem is that she is always with this Guy. Iam not jealous or something Iam happy for them but I cannot just avoid her all the time. So what do you think I should do?