I see your situation now. Don't be too sensitive and overreact. A man has lots of reasonsto be quite and not talk much. Maybe he is not in good mood but not necessarily because of you. Neither it means he is avoiding you. I guess he now must be distressed about your relationship and doesn't know what to do as you do. Put yourself in his shoes, you will know how hard it is. So, before you two figure out a way that make both of you feel comfortable, please give him some time and space, and for yourself also.
I'm sure you will work it out someday. But before that day comes, you had better identify what you actually want. Should you just want to be a friend and stay around him like you have never crushed on him? Are you able to watch he and his girl friend play around and you don't have a word? If your answer is "yes", then you probably need to learn to treat him as a friend from now on. A friend like any other friends you have, like 911, like 征哥, like me. You then cannot show any love to him, you should control yourself and hide your feelings. It would be cruel and painful. But it's the only way to keep your friendship and not hurt his emotion.
Or you don't want to be a normal friend of him. You want to be his girl someday if chances come. Then you probably need to find a way to gain his affection, to change his mind. However, I don't have any workable advice then. And since he has a girl friend now, if you do feel he and his current girl friend suit each other well or his girl friend is more suitable than you, then the possibility is minimal.
But as far as I see, the most possible solution is that your enthusiasm for him will cooldown as time goes. You are still young and there are many people for you to meet before you, of course many good boys. It's hardly possible to meet one's lifelong love at a age of 15. But I'm not saying you should fall in love with others, It's necessary to love and just enjoy it no matter it tortures or comforts. Anyway, all those will become the best memory of your life. At your age, I fell in love with many beautiful girls and thought they were my destiny. Now I know it's wrong. It's still to early for us to make a conclusion of our life. However, I never regret about my young love. Many years later, I can smile about what I have ever done.
Love is the most beautiful thing, you know that.