Your answer to your question is contained in your post. You say you feel you're "not enough" for him. Feeling inadequate is naturally lowering your sex drive.
I don't know where your feelings of "not enough" are coming from, but I bet it's from your childhood or past relationships, not your experiences with your husband.* I suspect as your self-concept improves, then your relationships will improve as well. I think the best thing you can do for your relationship is to get yourself emotionally healthier.
Get
Amazon.com: The Feeling Good Handbook: David D. Burns: Books This will walk you through identifying where these feelings come from and how to address the thinking patterns that are causing them. It also has very good guidelines on when to seek therapy. I don't think you're at that point yet, however. I'm a mental health counselor, and I've had people successfully work through depression and poor self esteem just by using this book.
*If your husband is undermining your self-worth, that's another matter entirely. I didn't see that in your writing, however.