I just don't know what to do to be more supportive of my W, when she is at our house she is miserable, she has horrible anxiety (she states because of my ex). She basically falls apart and I don't know how to make it better.
Am I tracking correctly that the general situation is:
You and your current W moved to be closer to your kid (good job, BTW!), and while the logistics (house, town, etc) are fine, the way your kid's mom interacts with your W is somehow messing with her -- even though there's nothing specifically concrete/tangible that your kid's mom may be doing. I.e., it's not like Ex physically stalks W or says specific things directly to her, but it's more nebulous. Your W is stressed out about this "can't put a finger on it" stuff in addition to the custody battle, and her perception is that you are also stressed out/consumed by dealings with your ex.
Is any of that close?
So I will say this, as someone married to a guy whose kids' mom has some challenging BPD-type traits (blaming, passive aggressive, manipulates the kids, etc etc, plus she remarried a uNPD guy, yay): it IS stressful and anxiety-inducing to be in a position where you're affected by a N/BPD's actions (whether concrete or intangible), but really not able to do anything to stop it. The stepparent role is one of a huge absence of control. If your W has PD-type traits (could be pretty mild / less than your ex, at least that is what I think I'm reading from you), then she will struggle deeply with feeling yanked around but not able to "do" anything to make the other person stop.
Are you two generally united on your perspective on what your ex is doing? You both see ex's behaviors as a problem, and impacting your life (though perhaps to different degrees)?
Am I guessing correctly that you are also stressed out about your ex, but at maybe a 4/10, and for your W, it's like a 9/10?
If you both are mostly on the same page that "wow babe, ex is such an anxiety-inducing person, she really stresses us out"...
I wonder if getting counseling together would help out.
Then it wouldn't single out your W as "you are the only one with a problem of falling apart under stress". It could be a "joining" thing: "Babe, all this

coming from ex is so draining... let's get support together, maybe we can learn some mindfulness techniques [or whatever] so our life in our home together has more peace for both of us..."
Not sure if you have tried that yet, but that's the first thing I can think of. And I'm in the thick of it, too, in that stepmom role, and it is a BIG challenge to try to let go and not let the ex "rent space in my head". DH and I have been married almost 8 years, and even today talking about kid stuff and the ex can be traumatic, and I mean that seriously. We still need a lot of help, and neither of us have a PD.
Feel free to share if this was close or missed the target...
kells76