well.... this is JUST a girl's point of view... so listen up... ;)
first of all, is that all that you want? sex? maybe she IS afraid of sex! but, can we make it one step at a time? i know this sounds corny, but kissing and heavy petting can be SO erotic. its a real turn on for girls, this foreplay thing. perhaps, without jumping the gun too quick, and going directly to actual vaginal-penetration-with-penis sex, you can try pleasuring her in other ways... like softly fingering her or giving her oral? because if she can enjoy those things... then she can enjoy sex, and she'll be smart enough to get it.
second, TALK TO HER. and from the sound of it, you probably have! many times! but i am wondering whether you REALLY talked to her, or you have just been trying to convince her to give it up and just have sex. you have probably already found out- that doesnt work. i say, suggest to her that maybe the both of you can take it slow and try OTHER things first and see where it goes on from there. Tell her that once the time comes, you'll both be ready BECAUSE you have been getting used to each others' bodies. having protection probably wouldn't hurt ;). believe me, the line "let's take this slow" works for us girls... well, at least to me, because it just makes me NOT want to take it slow. reverse mentality, anyone? I dont know, but i'm a sucker for it, I guess.