Hello KAW, and welcome. I am so sorry you're in such a difficult situation.
It's very clear that you care very much about this relationship and you've invested a lot of time and energy into it. You mention that, for your own health, you need to get out ASAP. What are some aspects of your health that are suffering? Emotional health? Your own self esteem? Anything else?
It sounds like you've left the relationship before but have been drawn back in. This is also common. A lot of people feel like they're saving their partner or rescuing them by sticking things out and taking what comes as part of the package.
Sometimes we end up doing what's called Caretaking - this is not the same as being a care giver and it's not something that actually helps the pwBPD. There's a very good book called
Stop Caretaking the Borderline or Narcissist - it might help you to take a look at that link and read about the book. It may help you discover some more ways you can step out of the drama and improve your own situation, which in turn will often improve the relationship overall.
How are you finding the lessons so far on the right hand side of this page? Are they helping you?