Hi Libby,
I'm so sorry that you are being put into a position of having to choose between your H and your family. That doesn't seem fair. I'm sure your family is very concerned for your own safety and emotional well being. It's hard for someone who is not in a relationship with someone with BPD to completely understand.
Has your H been getting treatment while you are separated? Is he open to working things out?
Could you share with your family a little about BPD and explain to them that although you truly appreciate their concerns that you do not believe that you are ready to end your marriage? When I revealed BPD to my family, I shared the video
Maybe It's More Than Just a Bad Day with them to give them a basic idea of what it's like. I also asked them not to mention BPD to my H but that I was sharing it with them so they can understand why my H is a little moodier, withdrawn, annoyed, etc. more than the average person.
One thing that might help is to also really begin working on yourself and ways to protect your own well being and sharing how you are doing that.
One thing that I don't do is share any of our fights with family. My H and I will make up, but my family doesn't get to see that make up process so if I were to share the things he says or does in a fight, then they only hear my side and would think he is an awful person. The only place I really share details of our fights is here. I don't even share details with friends. I might let them know that things are rough but I never go into details.