The Monogamy Jedi Mind Trick
Iím new to the polly world myself but from what I read it sounds like you may just not be ready yet. You have not made a deep enough emotional connection with your girlfriend for you to feel completely secure. So of course you are going to feel jealous. In your mind you are still wondering if you are good enough. Is she going to leave me for another guy? You know, the usual monogamy Jedi mind trick.
Until you are able to work through one-on-one with your girlfriend and are on solid ground then her dating other guys will not take you to the emotional brink.