My very best friend may or may not be bpd but essentially she has a lot of the traits including the pulling away and distancing. I too have bpd and from the beginning she's told me that she just "does that" and that she'll "always be back." This helped but of course she is aware of what she does. So what I do is try to remember this is just something she has to do from time to time and remind myself what she's said. And not freak out on her, even though I don't always succeed. I just wait and let her be and remember it's probably not something I did nor is there anything I can do about it in most cases. That's helped me get through the phases.

Just be patient with her and the more you are, the stronger your relationship will become. Don't take it personally and don't try to change that about her, she'll come back when the phase is over.

Hope this helps!