AIO to my nesting partner refusing to break up with our other partner that abused me and then dumped me? (polyamory)

hi, this is a burner account as both people i’m talking about know my main.

so to start, i 21F was in a throuple with my husband 23M and another woman 26F. We will call her Bella for the sake of privacy.

i have been married to my husband for two years, been with him for three, and we have a son together. last year we started a polyamorous relationship with Bella, and she is diagnosed with multiple mental illnesses including BPD and Bipolar.

the start of our relationship was great, she treated us both fairly well, had some insecurities she needed to work through from past relationships, but overall made an effort to stay on top of her meds, therapy, and even got sober from alcohol to help with her mood swings.

then things got bad. about 6 months into the relationship, she began treating me very poorly, she would fabricate stories about me that werent true and then force me to apologize for things that never happened. she constantly and consistently would accuse me of malicious intent no matter what it was that i was doing. for example, if i bought her a gift, she would accuse me of lovebombing her, or assume i was only buying her something to try and hide something i did wrong. if i complimented her, she assumed i was being sarcastic, or making fun of her.

if i hung out with my husband instead of her all the time she would break down in tears and claim that i loved him more than i loved her and that i hated her. then it would take hours of me trying to help her feel better and convince her that i truly did love her and him equally.

soon, she began bursting into the bedroom without knocking while husband and i were being intimate and would yell and accuse us of not using protection. it got so bad i did not feel comfortable staying at her house after that. but i was too scared of her outbursts to stand up for myself or set boundaries.

then it got to a point where Bella no longer texted me during the day, she never checked up on me, she would find reasons to get out of hanging out with me, and would only give her attention to my husband. she would buy him gifts, and take him out on dates, but when i tried to connect with her, she would be cold and dismissive.

the last straw for me was about a month ago when i gave her a call to ask how her night was going and she started yelling at me that i was purposefully trying to invade her quality time with my husband. (He was staying at her house that night.)

i then decided i was tired of constantly being villainized and berated and i said i was not going to apologize for something when i did nothing wrong. i have always had good intentions and have always done my best to be good enough for her.

she dumped me after i said i wasnt going to apologize, and said she was “convincing herself to love me the entire time”

now, i am not the type of person to dictate my husbands love life, and force him to break up with someone. but this woman traumatized me, and he refuses to break up with her. She never apologized for hurting me, and is now acting like nothing happened and we are best friends and is calling herself “mama” to my son.

i threatened to divorce my husband over it and he thinks that i’m overreacting and that i wasnt the most perfect partner either because “there was abuse on both sides” (he was referencing the fact that i lost my temper and yelled at her when she dumped me)

i don’t want to divorce my husband, he is a wonderful person, but i genuinely feel like she has been manipulating him this entire time and shes poisoning my marriage.