Living in a sexless relationship, what to do?

Ok, so let me get this straight. You two don't have a regular and active sex life due to complications in her life before you got together. You treat her well and do the typical love-type relationship stuff with her (taking her out, watch films on sofa cuddle etc). You take her past bad experiences into consideration, are sensitive to her feelings about it, and don't pressure her into anything despite the fact that after 1 1/2 years you're still only having sex occasionally. She wants you to initiate sex but when you do so 'at the wrong time' she gets annoyed.
Explain to her that whilst you care for her and want to work things out, you're finding it difficult to know what to do because she's not letting you know. The fact that she's not into sex and doesn't 'want you' sexually when you do see her isn't a good sign. If anything she should want you more after a long-ish period of time away from each-other. You need to clear the air with her because if something doesn't change, and soon, your LDR relationship with her could end.
Hope things work out but please talk to her