Welcome to BPDFamily, Dr.stormyweather! Your instincts are dead on. I was in your shoes with an ex boyfriend. No matter how much space I gave up, he demanded more. I agree with wrongturn - isolating is truly not a solution for either of you. Great job in sharing here and exercising. Keep finding ways to take care of yourself.
I feel trapped and don't know how to separate myself from her and establish boundaries.
Pace yourself and take baby steps. It took you 3 years to get where you are, it will take time to take back the space you've given up. You're posting here. You're exercising. You're in marriage therapy. Is there another step you'd like to take? She may not be happy about it at first, you can expect push back as you change, but this is worth it.
So proud of you. This is tough work but you can trust your gut!
pj