I agree with what everyone else is saying - sit down and try to have a calm, understanding situation where you make it clear to her that it's not fair to you if she can be with other people, but you can't. I don't want to scare you, but I don't want you to be blindsided either - so I'm just going to add that relationships in which 1 person is far more committed/compassionate than the other partner, they don't work out. Right now, you must be more committed because up until now you've been monogamous - but she's trying to 'keep you down' in the sense that she wants you to be more attached to *her* than she is to you. It's not okay. And what *could* happen and often does in situations like that, is the less committed partner gets bored and leaves. So stand your grand - either monogamy for both of you, or polyamory for both of you. Don't play by different rules, and try to keep tabs on her emotional investment versus yours.
It seems like she wants the door to swing only one way, her way. I wouldn't call her poly, but more like promiscuous. Hope you can live with that because that is the rules she has set and you seem to be adhering to.
Polyamory = having sexual and/or romantic relationship with several people.
Apart from that, you need to have talk with her about it. Explain, without attacking, that you feel like there is one set of rules for her and another for you. Explain that polyamory has to go both ways for it to work for you. Set some clear rules together about what is and isn't ok.
The two of you need to read a book or two on open/poly relationships if you want to try to pursue this. The Ethical Slut is a good place to start, I suppose, but there are a ton of other books out there.
It's time for the both of you to consider what you want in this relationship, and to talk about it with each other. Because if you're interested in doing things alone with other people but she's not cool with it, then it's not going to work. One thing that I like about the second edition of The Ethical Slut is that it has "exercises" sprinkled throughout that are designed to help you figure out what it is you want from a relationship, how to deal with difficulties that crop up in open/poly relationships, etc.
But, frankly, if she's intentionally guilt tripping you, I don't think she's got the maturity to handle an open/poly relationship.
(Also, despite what others have said, a relationship where one person is monogamous and the other is poly/open can work, it just has to be understood and accepted by everyone involved.)
I am monogamous with my boyfriend but he is polyamorous with his wife and myself. It works for us, but I don't guilt trip him or get jealous, and neither does she. These types of relationships can only really work if you have the personality for it though, and the maturity to talk about things instead of pointing fingers.