I agree that kconnors is right on!
A lot of her behaviors are defense mechanisms. We push people away because we don't think we're good enough, we feel out of control or that we're losing control, for me I always tell myself that the relationship will most likely end anyways so I might as well do it on my terms before I'm the one that gets hurt. There truly is no rationale for it, I wish I understood why my own mind tells me to think this way. Hurt or be hurt, it's a terrible way to live, it has ruined a lot of my own personal relationships, but it's just a mechanism of control for a mind that feels out of control.
Counseling is crucial. If her current counselor isn't working, she needs to seek another one. Once you find the right counselor, it's like they can see right into your mind. The first time I went into my therapist and she started explaining anxiety to me and asking me if I had certain behaviors and frames of mind, I started to smile. I couldn't believe someone finally could understand what was going on in there! Just the way she was able to articulate these things to me, I felt this burden life off my shoulders, a sigh of relief if you will. Was I happy to have a diagnosis, well, not really, no one likes to hear that they have a categorized mental issue, but I was finally able to tell myself, gosh, this is actually manageable and there are other people out there that think like I do!
So the best thing you can do for her, is be understanding! Have you ever thought of reading up on GAD? I find I open up most to the ones closest to me when they understand me. It makes me feel less judged. My best friend suffered with panic disorder as a teenager which I never could understand until I started to develop GAD in my twenties. Now, her and I are the only people that understand one another when she's up at 3am and her heart is pounding because her bf didn't return her text message or I'm patrolling 0409, over-analyzing every single comment or 'like' my bf posts. Anxiety is usually paired with some sort of insecurity, childhood trauma as mentioned, the feeling of needing to be in control and not be viewed as weak (for me personally at least). So when dealing with someone with anxiety, come across as least threatening and challenging as possible. But as kconnors mentioned, you also need to make sure you're taking care of your own needs. It will never improve unless she seeks the help she needs.
Maybe someday I will be able to take my own advice! It's always easy looking in but just remember that her reality is askew. Her world is not as you see it.