I completely understand where you're coming from. I'm mono, but I've been in a relationship with someone before who was poly.
One key to allowing yourself to feel comfortable with "sharing" is to realize that a person isn't property, and honestly, your partner is far ahead of many people who claim to be polyamorous because she was forthright with you.
Another issue that arises is when the two partners she has dislike each other, or don't spend time together. It's easier to tell yourself she's with one of your friends than to be angry because she's run off to "the other one" again.
Have you spoken to her OSO yet? It might help to ease some of the tension you've been feeling... break the ice. As long as he knows and is fine with it, I think you guys could have a good road ahead of you.
I'm not saying it will be easy- I've been there and broken up for various reasons- but even as someone who is mono I can understand the good that comes from having multiple loves.
Honesty and communication are going to be key. Between her and you, her and him, and him and you.
There are some good books that can help as well- such as "Opening Up" and "The Ethical Slut." They really assisted me in letting go of some of my fears and jealousy in the past. Good luck!