Are you sure she is ND and the lack of sex isn't about something else?
Have you read "Divorce Busting" or "The Divorce Remedy"? Another good book is "The Five Love Languages" by Coleman. Perhaps if you concentrate on other areas of your M, the sex will improve. I think it would have for me. Strange as it sounds, I think my STBX withheld sex because he was angry at other things.
You W may not even know why she doesn't feel like sleeping with you.
