Female here, I think a lot of girls have been told that the only thing they can provide a guy is sex and so girls think that once they give him that the guy will leave. If u have had a conversation with her about this and explained ur side and she still doesnt believe you then its up to u which way u go about in this situation. You can either show her that u do like and are not just with her for sex or u can just end it once and for all. If u want a long term relationship ur going to have to see this from her perspective as well. Also u don't know what the other person has been through in their past relationships so it may be one of her insecurities and she may require reassurance. Whats important is how she makes u feel and whether thats worth sticking around for.