Welcome to the group! You are in a good place of help and support. My story is similar to yours. My step daughter, now 22, has BPD and has lived with us on and off since she was 18. She typically stays with a boyfriend until the relationship is over and then she stays with us. If our relationship is tense, she will go to live with her mother but she can only stand that for a week or two (her mom has BPD, too). We go through really rocky times but sometimes it's better.
When you find the right therapist (and I hope your partner does, too), that will help a lot! I learned to set boundaries (sort of,
-it's a work in progress). I work really hard not to get sucked into the drama, although sometimes I still do. My therapist taught me a lot about mindfullness. I read a lot to help me understand what she's experiencing and how I can support her while protecting myself. Taking care of myself and my own emotional wellbeing has been the most important thing for me. Some books that helped me are
Loving someone with borderline personality disorder by Manning
When your daughter has BPD by Lobel
Stop Walking on Eggshells by Mason
My mantra comes from the 3 C's of having a relationship with someone with an addiction, which also apply to loving someone with BPD: I didn't cause it, I can't control it, and I can't cure it. I work really hard to give her the space to make her own decisions and face her own consequences. I give her limited advice when she asks for it but mostly keep my mouth shut unless it involves me. Our conversations mostly focus on making healthy decisions and thinking about consequences of her choices. Her dad and I both have occupations that can be really emotionally charged and have learned to regulate our emotions when things are stressful. This calmness when she's upset does not add fuel to her fire, although it sometimes irritates her that she can't provoke us. This emotional turmoil and intensity can be really tough on all of us.
It helps that her father is supportive of me and willing to set firm boundaries with his daughter. Nastiness and disrespect are an absolute boundary and her father has made it clear to her that she can find somewhere else to live if she treats us badly. I think that's what has made the biggest difference in her behavior. That all being said, every situation and relationship is different and my stepdaughter doesn't seem to have the ongoing rage than many others with BPD do. Her anger is mostly directed at herself and only occasionally at us.
I hope that you will figure out what will work for YOU first. The emotional intensity can be overwhelming and destructive to your own mental health and relationships. You can only really be there for your partner and step daughter if you are in an okay place yourself, and the same goes for your partner. It's really easy in relationships to allow the child/adult child with BPD to come between a couple. They can suck all of the oxygen out of a relationship with their drama, demands, and manipulations. However, if they can see a loving relationship with appropriate boundaries, this can help them to see how a healthy relationship can work.
