It seems like one of the purposes of sex has been lost between the two of you. Sex is a form of communication--and communion. In it's best moments it is an affirmation of love and trust. While sex can be just plain old fun ( an important part of adult play) if it only becomes a way to satisfy a sexual itch- or placate a partner- it loses all its magic.
Things have gotten too mechanistic. I think you need to bring romance and intimacy back to your sex life- otherwise this demand-withdrawal squence is going to kill the relationship- and it sounds like the relationship has other strengths that you might not want to lose.
Given that you have expressed some of this to your girlfriend- to no positive effect- it might be a good idea to go to formal counseling.