I too had this problem for many (over 20 - just sad) years - masturbation was not a problem at all, but I just couldn't with my husband. (Or anyone before that either, believe me I spent several years looking for the man who could get me off )
I finally began to realize that my own self-conciousness was the problem - I was afraid of the having those strong physical and emotional feelings in front of him. Then, as I thought about the problem some more, I acknowledged that I had always held a part of what makes me "me" from him - refused to be vulnerable or afraid or soft or weak in front of him. I wanted him to think of me as strong and capable and independent.
So, having gotten to that point in my self-analysis (contemplating my navel ) all of a sudden my barely acknowledged and shamefully hidden submissive tendencies began to rear their (blindfolded!) head! As I mentally began to accept this part of myself, I began to relax and become more trusting and open-hearted and willing to be vulnerable with my husband and occasional orgasms began to follow! Yeah!
In the last few months, I have begun, very tentatively, to share my submissive desires with my husband. He has agreed to begin traveling this path with me and the few times we have "played", my orgasms have been amazing!
I guess for me, the problem was me! As I let go of preconceived notions of "proper" sex and worries about how I appear to him and fears about him thinking I'm weird or gross or sick or whatever - in other words I've begun to trust and accept myself and him completely - orgasms have just begun to fall out of me! Oops!
I could talk for hours (snore) about my sexual journey the last few months, but I'll just say don't give up. See the doctor, spend more time learning your body, trust yourself and remember - any woman in the throes of passion is beautiful to her partner.