She doesn't deserve your love. She sounds like a prissy bitch who thinks life is a fairytale and wants two princes to fight over her. The only way you will get the upperhand in this situation is to make her realise that you will not wait around for her while she goes and makes out with another guy. Seriously, if it was the other way around (you with another girl), do you really think that she would do the same for you? Face the facts dude, you're just the rebound guy that she hangs around when she's not with her boyfriend. I would never cancel an engagement I had with my friends just because I was a little tired from hanging out with my boyfriend prior to that.