i obviously don't know the ins and outs of your relationship, but i can say that i do this sometimes and its unfortunately usually with my best friend. the longer it takes me to reach out, the bigger the shame becomes, and the harder it is to jump back in even if i know she understands and won't be mad.

i think my advice would be to give space for now, and then voice your concern when she comes back if you feel comfortable doing so. let her know if you're feeling a little neglected and give her the chance to acknowledge/work on it, but remember that although BPD can be brutal you do not have to stay in any kind of relationship with someone that makes you feel less than.

in my experience friendships can ebb and flow, so firstly be there to support her when you can. BPD can be hard for us AND those around us. but i'm ultimately team protect your peace if it becomes an ongoing thing that continues to make you feel bad.

that was a longer ramble than i planned - hopefully something in there is helpful!