Think about what YOU want. This is especially important to realise as a poly person, not to be pushed around or controlled by one person and particularly someone who doesn't have your best interests at heart.
Your feeligs do not control you. Sometimes you DO have to break up with someone who is toxic for you. I have had to end a couple of emotionally abusive friendships and one good relationship for that, where the person refused to accept me for who I am (specifically the poly thing). You are worrying about this other person you're dating vs the needs of your ex. Think about yourself instead. Only by knowing what you want first and being true to yourself can you be there for or commit to anyone else. I've learned this recently. My otherwise-perfect ex (actually in hindsight you always realise things weren't 100% perfect) could not accept who I am but it is good that I broke up with him. You should really do what is best for you or you will never know what you want.
Take the plunge, do something just for you even at the risk of other people not getting what they want, even at the risk of missing an opportunity (trust me there are so many other fish in the sea. After my previous breakups (I've been mono up until recently) I've always thought I would never find anyone so amazing as that person and when I found someone better they made me wonder why I'd dated these guys in the first place and how I could have thought they were so wonderful or so irreplaceable?
To me your ex seems selfish and does not want you to be happy but only to fulfil her needs, which also means she does not appreciate you for who you are but only as someone to serve her insecurities. Who wants to date someone like that?