Hello and welcome to the forums, thanks so much for sharing and I am so sorry for what you're going through. You're right, it takes a lot to post the first time but it was a very good decision since others can understand what you're going through.
With BPD, there is an overwhelming fear of abandonment that can cause some serious turmoil in relationships. In a nutshell, the BPD is so afraid of breaking up that they begin to lash out and cause turmoil in the relationship...which often leads to a break-up. Or sometimes they'll leave so you can't leave them. It doesn't make logical sense but that's the whole problem here- disordered thinking when highly emotional.
Of course, you don't have a diagnosis (yet) and that's more for the clinical aspect anyway. Your goal to smooth out this relationship is to learn better communication techniques, how to validate her feelings and make her feel secure, and to get past these highs and lows. The good news is that BPD or not, this site is invaluable in teaching you those techniques and it is helpful in all relationships.
Please take a look in the Tools section at the top of this page or through this link-
https://bpdfamily.com/message_board/index.php?action=post;topic=3059825.0;num_replies=0#Again, welcome to the family and please ask as many questions as you can. We're here to help!