Hi sickandtired24,
Welcome. You've landed at a really good place. Some of the things you've described resonate with my experience as well, such as the lack of boundaries, the superficial relationship (to protect yourself), the high conflict personality, the inability to apologize, the need to blame others to avoid ever looking inwardly, being unlovable, dragging up old history (distorted) and the fact that she can make you feel like nothing you do for her is good enough, not making good decisions (an inference of mine since you mention she's been arrested)...
There have been many times during her 'meltdowns' that she says she knows all she needs to know about therapy and that she doesn't want to do it anymore. How can you help someone who won't even help themselves?
You are right. Since she is in her mid 30's, but still living at home (with your mom and dad) and with her husband, and having daily meltdowns, how is that working out for your parents? It doesn't seem sustainable...
At what point is walking away the best option? How can I support her but also not sacrifice myself in the process?
I am wondering if you have you had a chance to read "Stop Walking on Eggshells" yet? If not, it's a really
really great place to start. You will find so much support there, and it's easy to read. Many people find that once they get started, the book is so validating to their experience, that it is hard to put down.
I would suggest reading something like this, before you "walk away", if you can. It gives perspective, practical suggestions, and after reading it, one can decide with a more informed approach, as to whether "taking a break" or trying some new strategies is the best way forward. On the other hand, if you feel physically or emotionally "unsafe", then it is a good idea to take a break from her. Keep in mind that breaks don't need to be permanent. They can be temporary too. BPD's are high conflict, so part of our challenge is to learn how to not be reactive to the craziness, and not be drawn into that.