Hi Arizona
Welcome.
A lot of what you mentioned is close to my experience and others here, so I hope to give you some thoughts.
I had described to a friend the first 8-9 months of our relationship as one of the most stable and happy I had ever experienced.
The first few months of my own relationship with the pwBPD partner were very happy, but almost too happy when I compare it with my other experiences.
When he asked me to move in with him his personality started to change.
Often, BPD traits aren't expressed enough until a certain closeness is reached.
I don't know who i'm going to get hour to hour, day to day. One day I'm the love of his life the next it's "I can't to be around you" if we have a disagreement.
I know what you mean. Many nons share a certain state of emotional exhaustion as a result of being on constant alert to these mood changes. The fights my partner and I had were terrific and could occur sometimes a few times a day.
Often he is quick to leave te relationship by comments such as "I'm moving out" and most recently going out and putting an offer on a house for himself just a couple of days after we had a healthy agreement to buy a home together... .he later flip flopped several times until he cancelled the offer when he saw that i was ready to leave (i have never talked about leaving the relationship in the 3 1/2 years of being together).
Jumping straight to the extreme of terminating a relationship is common among many BPD partners. My own partner brought up termination more than dozens of times, and walked out many times also. I don't think a healthy relationship involves using the status of a relationship as a bargaining tool to get what one partner wants.
Why am I still here? I love who he is when not in his mental health illness... .i fell in love with that man i had the first 8 months.
You're not alone in this.
I hope to hear more about how this develops for you.