When I first met my poly partner,he was in a relationship with a totally 100% monogamous women who wanted the whole works,kids,marriage etc. When she realised that wasn't going to happen,all hell broke loose,temper tantrums,threats,you name it and eventually some of that was directed at me,his new partner. That ultimately cost her the relationship because she wasn't meant to be with a poly man(her own words). From my perspective(I would describe myself as poly-curious) I was able to accept my partner's choices not just because I truly loved him and wanted him to be happy but because I also loved myself and was secure enough to understand that this wasn't just about me. Sure I get jealous and sad but it is much less now that we communicate better.
To me,coming from a mono perspective,it sounds like Bree truly is a 100% mono and doesn't want to share you with anyone,its just how she is and if she wants something you can't give her,then you have to set her free..
I wish you luck!