It was a lot like learning a foreign language. 
It is a foreign language.  People who suffer from BPD process information in ways completely different from the way you and I do.   Actually it felt a lot like my Ex and I were always 180 degrees apart.   When things felt calm and ordinary to me, and I was starting to relax, things felt empty and void to her and she needed stimulation so she would stir things up.  When we were together and sharing I thought oh how sweet this intimacy is, and she thought how absolutely terrifying this is I let my guard down now I will be hurt.   When I was troubled or upset, she thought now things have gone badly wrong, I can't deal with this, I will push her away.  When I couldn't soothe her feelings she felt I was deliberately holding back and that made her angry.