I feel like my girlfriend doesnt care about me?

Me and my girlfriend have been dating about 14 months. In that time I've treated her better than anything, I've put her first almost no matter what, she isn't a slut or anything but last year she switched from an all girls school to a coed school and hangs out with girls that give guys head in the bathroom. I don't know why but I always feel like she's going to cheat on me. once while she was in the bathroom I read through her texts and there were more messages from other guys then there were from me. I've told her I love her an she said it back but she won't say it unless I do first. she won't actually say anything that has to do with anything emotional unless I dofirst and then she usually just repeats what I said. most of the time she treats me like crap, for example, I got us tickets to a concert and she tried to uninvite me for one of her friends. Please help I don't know what to do, I can't break up with her because for some reason she means too much to me I just don't know what to do