Just my two cents, calm down. No one is telling you just up and divorce your husband. Guess what? Some of us are married and DO take those vows seriously. Also, we take our other relationships seriously, we take ALL our commitments seriously! Who'd a thunk??
If I were you, (and again ignore me if you feel like it, no skin off my nose), I'd worry more about how things are happening between you and hubby. I have a somewhat similar situation, but it's not metamours. It's family.
I am working with my therapist about confronting negative emotions because I have a hard time with it. My mother is EXTREMELY abusive and since she flips switches, when she pulls crap and then the next day acts like nothing happened, I tend to do the same. Thing is, that does not work for hubby.
So right now he is telling me that he will be confronting my mother over issues that need to be dealt with, he is very upset. Just because decades of abuse has taught me to back off and ignore with my mother, that's not healthy and it doesn't work for him. So I do NOT try and convince him to be okay with things. He's not, he has a right to not be. The fact that your husband not only refuses to respect your boundaries of not having her in YOUR life, but tries to get you to forgive forget and move on is an issue between YOU TWO.
Me: Late 30s pansexual poly.
DH: My husband of 19 yrs and father of 3 teen girls.
DC: LDR of +4 year