15, is young, but i feel that you like this girl very much, but you need to find you another girl, if she is treating you this way you are young yet, and will have many girlfriends before you get your education, also you are both young so date others you may find out you like another one better and remember you have lots of time to date try and worry more about your educatin and try to control your emotions if not talk with a school counsler i wish you the best luck jo
At this age relationships are rarely forever; although some kids are happier than others to skip quickly from one relationship to the next. You need to talk to your girl about this situation, tell her that it is not cool with you. She may be happy seeing two guys at once, but that doesn't make it OK. She could be being selfish here, doing what she feels like without any concern for the feelings of you (or the other guy, for that matter).
Tell her that she needs to make a decision. If she wants to be with you she can't be seeing other guys at the same time. If she doesn't want to make a commitment to you, then it's over.
It's not unusual for some people to feel that, until they have made a specific sort of commitment to one person, it is OK to continue playing the field even if they are seeing one person regularly. This may be her attitude. That attitude is only OK if everyone involved is OK with it. You are not happy with this, and you need to tell her that she can't do this to you.
I can't predict how this will turn out, and she may react by deciding to finish with you. But either way is better than going on as you are at the moment, so you need to have this conversation with her.