I grew up in an abusive household that used polyamory to justify some of my parent's behavior. My partner is poly, but when they tried to date a friend they've known for years, she became violent and abusive. I want to be rid of my trauma, but now I have new trauma. What do I do?
I am so sorry to hear that you continue to experience trauma at the hands of people calling themselves polyamorous. The first thing I can suggest here is therapy - preferably with a trauma-informed therapist who can work with you on the abuse and trauma from your childhood.
The next step is to talk with your partner about this. It sounds like this second trauma was something you and your partner went through together after your metamour became violent and abusive. What have you two done to heal together? What do you need from each other? Consider attending therapy together, or helping her find a therapist to see as well.
You could also do a mini book club where you read a book together about trauma survival or healing from abusive relationships and talk about it. Some partners find it really powerful as a bonding experience to read out loud to each other; or you could just read it independently and talk about it afterwards.
It is OK if you need to take a break from polyamory for a while - it sounds like you have two pretty significant traumas associated with it and you may need time to heal. This is something you can ask of your partner - "hey, while we're working out the abuse we suffered together and figuring out how to heal from the past and how to build boundary-setting and red-flag-recognition skills to keep it from happening again, can we temporarily suspend the addition of new partners?"
Your partner may be unwilling to do this, which is their prerogative, but then you'll need to decide whether it's critical for your healing to step away from this relationship if that's the only way to step away from polyamory. But don't go there just yet - start working with your partner on healing.