Im a guy, but here is something to think about.
Has your sex life always been good together? Or did it start off a bit shaky?
The reason I ask comes from personal experiance, my wife and I have an absolutly fantastic sex life together nowdays, but only due to me hanging in there, and my wife putting in a hell of a lot of effort!
The first few years of our sex lives was terrible, and a part of that was constant sexual rejection on my initiation, this went on for years, and in a nutshell, has left me very wery, and to be honest, still absoutly terrified to initiate sex.
Maybe this is something from a former partner?
Nowdays my wife and i talk openly about the issue, and all sexual issues, which is helping me overcome my fear of rejection.