You need to examine the sources of your sexual desire. Are you more turned-on when the person is either new or not too serious?
A Sex Therapist Can Help
Yes, I agree with your ex-girlfriend that the sexual connection can be helped by therapy. There are lots of specialists in sex therapy who will either meet with you in conventional settings or in retreat-like sex weekend sessions that really focus your energy on each other and your sexual issues.
Sex Can Go Bad In Serious Relationships
I think that for many of us men sex is easier when it's not too serious. In other words, the more fun and non committal it is, the better it can be in terms of performance and lack of anxiety.
However, as the relationship becomes more intimate, the sex can feel too real or not as "fun." You may be starting to think, "Is this going to be IT for the rest of my life?" No, Virigina, there is no sexual Santa Claus. It's true that once you commit, there will be no new partners
But, as you will learn in therapy, sex with your significant other needn't be boring or routine. There are thousands of ways to spice up your sex life from creating new avenues for intimacy, to trying all kinds of new positions, to role playing or use of toys.
I really believe that it is possible to have a fantastic soul-mate and fantastic sex at the same time. Not only is it possible, it's the greatest way to live!