All terrible things must come to an end......... thanks all for your advice and info. The hubs and I had a talk last night and he agreed to cut off contact with the Cowgirl. Both he and she agreed they wouldn't be okay with "just friends" and especially with a more distant friendship (which is all I could personally deal with without freaking out-- I've been having minor panic attacks over this whole thing lately).
The only problem left is my stomach-wrenching sense of guilt. I feel like I needed to make this call for my health, my marriage, and recovering from some major damages from this whole experience, but I can't help but feel that it wasn't MY call to make.
Hubs agrees that my view is mostly logical, though from his perspective her behavior has been a lot better than it was before and she deserves a chance to be in his life for another try. But he knows that the cycle of abuse is hard to break and it is a risk-- and that his main priority is us and me being happy.
So... I guess it's "over"? But he's sad. He loves her. He promises me he'll get over it, but this IS hard for him. And not only do I feel guilty for shoving her out of his life IF she is truly "getting better" but I hate that I've hurt the one I love/prevented him from continuing a connection with someone he cares about. I guess (this may be petty) I want HIM to feel like this was a good decision... but I can't change feelings, just dynamics.
I'm sticking with my boundary, but it still sucks.