I don't like sex anymore

I'm 20. And I just don't like it. I still have a bit of a libido, mentally at least (I fantasize and whatnot) but anytime my boyfriend makes things go that direction, I just feel uncomfortable and want him to stop. When he asks why I feel so bad that I can't give any reason other than I don't want to. It's really not just a case of 'not being in the mood' because I haven't been in the mood for months. We still do things, but only after I've worked up a lot of determination, and basically every time I feel just relieved its over and like I'd have been happier snuggling and watching a movie or something. I'm totally confident its not his fault. I realize the obvious answer would be I'm not attracted to him, but that's not true. I fancy the hell out of him and I don't find the thought of being with anyone else any more appealing. He's totally considerate in the bedroom and always tries to show equal consideration to my 'needs,' but I just don't feel anything. It's so frustrating. I know the problem is in my mind and my body, but I don't know what's causing it. Basically, that's my question. What could have caused this? I never felt this way a year ago. I don't know if its a stress problem (I have been under a lot of that) or a health problem or maybe a sign of some kind of mental condition. So far I've basically just been pushing through and hoping my sex drive would come back in time, but it doesn't feel like it is. Does anyone have any experience with this and how to deal with it? Most of the pages online I've found seem to put the cause down to middle age, stale relationships or childbirth. Which doesn't make me feel very good as a young woman in a fresh, exciting new relationship. :(

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