Welcome Book Worm. Coming to the realization that your significant other may have BPD is both a relief but also scary. On one hand you finally get to understand why they act the way they do, but on the other hand, it means there may mean they will never get better (although some do).
All of us on this board are choosing to stick with our pwBPD despite knowing how difficult that will be. We understand that it will take lots of hard work, that we will have very challenging and difficult times, but we can see the good in our pwBPD enough to know that the good usually outweighs the bad. We have a lot of lessons to learn new ways of communicating and taking care of yourself. That might be a good place to start.
Here is a link to one of our workshops on the Dos and Don'ts of a Relationship with someone with BPD
https://bpdfamily.com/message_board/index.php?topic=62266.0