it sounds to me like your girlfriend is going through a pretty confusing time in her life right now. not just in relationship to you but maturing and becoming a women. being eighteen is a exhiliterating and confusing time for a women (believe me i know i am going through that same thing right now).
i wish that i could say one thing that would "fix" this problem that you seam to be having, but unfortunately that isn't how it works. but what i can give you is. having sex at this point in your relationship should be more than just getting off, it should be magic. it should make the two of you feel like you are one, and that nothing in the world matters except the here and now. based on what you have said she doesn't feel that. my question to you is why?
i dont doubt for a second that you love your girlfriend or that deep down she loves you. what i can say is that she is probably waiting for you to reignite her passion. when you truely love someone you can tell when they have ulterior motives to the things that they do. and as much as you say that she is being selfish so are you. try understanding why she feels the way she feels, and not just about the sex issue. try doing something solely with her in mind. want nothing in return. talk to her, communication is everything in a good relationship.
i wish you the best in everything that you do and i hope that the two of you can come to a deeper understanding of each other.