As I read your post the thought that you might be suffering from fear of intimacy with the opposite sex came to mind, hence why you probably have a low sex drive and prefer masturbation to regular sexual satisfaction with your significant other... men are at their most vulnerable emotionally when they are having sex and they often feel deeply during this time even if they do not desire to do so.

Is this a possibility for what you are dealing with?
Have you ever heard about Sexual Anorexia? if not look into it.