One simple thing that helps is coming to this forum. By communicating your anxiety, you are more likely to relieve it. It also helps to know that jealousy is common and that many people can move beyond it. I would say it's a natural response, but some lucky few (like myself, thankfully) don't experience jealousy. In any case, a lot of what you might need is to understand why her actions bother you. It sounds like she's not taking time or intimacy away from you, so there's little that is being denied.
Instead, you might have to look at what your image of how things "should be" versus how things are. Then consider what you are willing to handle and also compare that to how things are. It may be helpful to ask yourself what the worst case scenario would be like and then how likely that scenario is.
Often I see partners who are worried about poly folk because of their mono programming. In mono programming, love is finite. Poly wiring is different. It may help if you understand that poly folk have the capacity to grow their love rather than being limited. Thus, if your fear is based on a lack or loss of love, poly folks generally don't work that way. Loving one person doesn't require a poly thinker to take away love from anywhere else. It's perfectly reasonable that your girl will love you all the same regardless of this other guy. For me, I love and adore my mono all the more because I can talk to her about others and how they are good people and share my worries with her. Having my mono's support is wonderful for helping me be a happy poly, and I don't ever forget how special that is.
Best of luck.