A difficult situation, and I am not going to contradict the advice you have been given so far.
I understand that this woman is fantastic and that you are in love with her. I understand that you are monogamous and don't want more than one person in your life. You also are sure that you don't want to share anyone - you want a monogamous relationship.
She is not going to change her nature for you - if she is truly poly, and she left her husband, chances are she wouldn't be able to be monogamous with you - if she has the capacity to fall in love with multiple folks at once and want that simultaneous relationship, then she is going to want that while in a relationship with you, too.
So I think that you have to ask yourself if you are happy accepting this the way it is, and things not changing. It's not a "best of all possible worlds" happy, it is more of a "net happy" - does the happiness you get from this outweigh the pain you sometimes feel that she isn't only just with you? If it does, then carry on, knowing the decision you made. If it doesn't then this really has no future and why are you putting yourself through this?
I'm not a big fan of the advice that there are "plenty of other fish in the sea" - some people really struggle to find folks that are compatible. I just think you need to look at the here-and-now and make a very honest self-assessment.
Please check out The Birdcage - an open, friendly Polyamory forum for all parts of New York State
"Listen, or your tongue will make you deaf." - Native American Proverb