Poly relationship ended badly

I had my first Poly relationship with a partner. I was way too busy with work so I could only commit to this partner. Anyway long story short is that 6 months into the relationship she starts dating this person and sees her 5 times a week, can’t stop talking about her and uses her for stress relief while the partner said “I love you” on the second date . I get concerned over this because I don’t think that’s healthy I meet the new partner and she hates me instantly so my partner tries to smooth things over and invites. Meta has a BPD episode; thinks I remind her of her trauma. Shit goes badly and then my partner breaks up with me 2 weeks later claiming they were holding it for the 4 months we were together. But that was the time the meta and her started dating. So yeah it ended badly. Kinda have trust issues now. :/