I don't have a whole lot to say that others haven't said. I think you'll find the general consensus is wide spread of your issue to people to have and haven't posted.
Ma grrl rpepper said the most important that I feel needs to be emphasized...
The first lesson in poly is to love yourself and she doesn't. She can't possibly love others without that.
So i'll reiterate it.
She needs to learn to love herself.
Loving yourself is the complete and utter approval and acceptance of who you are in the world. The void she feels inside isn't filled by the "stability" she's found in you, or the "passion" she finds with these other guys. She feels void because she doesn't love who she is. She may not even know who she is to begin with.
We spend so much of our time waiting to be loved, hoping love will find us, searching, yearning for that special love. Feeling empty and lost without it. Wanting someone to give us love and fill us up. Unfortunately, thatís not usually how life works. You will draw to you exactly what you create in life, and what you believe you are worthy of. So loving yourself can create love in your life. When you expect love from an external source, and someone or something does not fulfill your void and fantasy's, then you will feel worse than before. Love may come for you. However if you are mentally unstable in the insecure sense, you may not handle the relationship correctly and you will lose it. The love and admiration is meant to be shared. Not a greedy fill for your personal issues; and personal issues are abundant.
She needs counseling...done deal. When she deals with her own baggage, you guys can evaluate if you can really be with each other or if you should move on.