It seems that you're trying to justify how good of a boyfriend you've been to your lady, and I don't think that's the issue. Wanting to love someone else too doesn't mean that you've somehow been unable to fulfill and satisfy her. It sounds to me that she does love you and is wanting to share more of herself with you than would be possible under "normal", mono circumstances. If she can be free to be herself and share her feelings with you when she loves someone else, she is sharing her true self with you instead of trying to fit the mold of how she "should" behave and feel.
I agree with Mono that you shouldn't break up with her now; that seems to be jumping the gun. If you truly love her, then try to understand why she is seeking this freedom. This will probably take a few awkward, heated conversations to get to a mutual understanding, but if you can work through your problems, there is probably a happy, healthy relationship on the other side.
As with most relationship issues, you probably just need to work on communication. Why is being poly right for her? Why isn't it right for you? A wonderful thing about being poly is that you get to define where your boundaries are. If you can express to her what you're feeling and rationally why you feel that way, I think that'll get you pretty far.