Panda, kudos to you for being open-minded and level-headed about this and being willing to consider the advice of other people on this forum.
As a poly female, I am able to love more than one person and it isn't just about sex or using people for sex and money. Believe me, I could use people like that if I wanted to, and I've had some guys actually ask me if I would just because they wanted to be with me. (I refused, because that isn't the kind of person I am or want to be.)
You two definitely do have different perspectives, but it doesn't mean hers is bad, just like it doesn't mean yours is bad. Some people are mono, and some people are poly. The best way is the way that works for each individual person. There is no blanket best way that applies to everyone.
It honestly sounds like she might be poly, but is insecure about it and isn't sure how to bring it up to other people. This doesn't excuse her behavior, but it might be a starting point to both of you being able to grow. If she learns from this that being dishonest doesn't help her to get more love, she might start fostering a habit of being more honest and up front. It usually isn't too hard to find fellow poly folk if you do some digging around and make it known first and foremost when meeting new people.
I wish the two of you the best of luck working this out. It sounds like there is potential for great friendship relationship there, even if a romantic relationship is off the table. And if she truly is poly, then she should probably have a focus toward having all of her relationships be positive relationships in her life, romantic or otherwise.
I am glad we made you feel safe to share and be open with us.