I agree with everyone else, he is one of those who will cheat on you. If you act like you are not bothered, and start seeing ither guys, I bet that he will get jealous then and want to start 'seeing' you again. He wants the best of both worlds. Just forget him, it might seem hard now but it will get easier. And if I were you I would text him and tell him he needs to sort his head out. It is also very unlikely that him and his girlfriend will last, she will eventually see him for who he really is. Just enjoy yourself, have fun! Don't get tied down with one guy at 15, you've got the rest of your life to do that.