What is wrong with me?

My fiancé and I have been together for 5 years now. We "had" a great sex life for the first 2 1/2 years of our relationship. But since then, we will go up to 8 months without sex. It's not him, it's all me. He wants it but i don't. I don't what is going on. I love him, but just can't get in the mood for sex with him. What is wrong with me? 
Any advice?