How exactly do you let her know you want sex? Personally, I never say a word, I just start playing with her hair, touching her neck, stroking her body and kissing, and before long she jumps me.
If she's pushing you off when you try this, then I'm afraid it may be time to reconsider your relationship. A relationship without sex, or at least some intimacy, is basically a friendship. I know this is going to seem harsh, but it sounds like you have become her platonic boyfriend. She probably cares about you, enjoys your company, but doesn't feel the sexual chemistry with you any more.
I say this because I've pulled many girls from relationships exactly like this and revived their so-called low sex drives without much effort. I could be wrong and there could be a genuine medical reason for her low sex drive, but you may need to reconsider your options before she does.
However, before making any rash decisions, try talking to her about it and tell her your thoughts.