Sex issues

Ok you girls, I have a problem. 
Is it me? My boyfriend and I been together for 1 1/2 year he's 22 and I'm 21. 
My problem is I just can't have sex, like I don't get in the mood when I'm with him he can do everything and I won't get turned on and I try and get my self wet but I loose patience and say "next time" it's sooo frustrating. He isn't a virgin, I am though and I just don't know why I can just have sex? When I'm alone I get all horny and want too but when I'm with him it's so different.