I'm three months into a relationship and I am the partner with BPD.
Unsurprisingly, we struggle already, and my lack of trust is the primary issue. Building that trust is another topic for discussion, but I want to share something that we do that seems to be helping me come back to center a little when I go off the rails:
Since before our first date, my guy has been tracking our relationship with near daily selfies of him that he texts to me, photos of us when we go out, and a spreadsheet of our dates / events. He posts all of this online for us to access privately. In doing this, he has created a special and tangible place for demonstrating the love in our relationship that I cherish. He is quick to capture ANYTHING positive and logs almost nothing negative. Further, when we are in turmoil or we break up, he has agreed not to remove or limit my access to any of it.
How this helps me:
1. When I go off the rails and turn the whole world black, I can look back and see tangible proof that we have good times too, and that we are planning more good times
2. Having a timeline of our dates/events helps give me a sense of consistent, objective reality (if that makes sense)
3. When I experience that HORRIBLE panicky feeling that he's cheating on me right this minute!, I can see in his face how much he cares for me via those pics
4. I can't deny the expression of caring and effort he's gone to in order to create and maintain this special place for us.
5. Knowing that I will always have access to this place gives me comfort. Even if he leaves, I still have all of this--it is proof that I'm cared about.
6. While I hate pics of myself (surprised?), I can't deny the happiness I see in my own face... .
P.S.:
1. Perhaps this would need to be a 'read-only' place for the person with BPD so that s/he can't sabotage it in times of churn.
2. I know that this idea is challenging if there is little positive going on/LOTS of daily churn.
3. I get that believing what is captured will likely be disputed, but if the BPD person contributes during 'white' times, maybe this would help?
4. I can also imagine that something like this could be highly inflammatory during dark periods and provoke a person with BPD to act out (trust me, I GET that)... .but over time maybe it could contribute to a solid, consistent foundation for the relationship as well as provide a safe haven (after the BPD person has jackhammered the hell out of it first

)
anyway, just a thought. It's a great way to honor our relationship and seems to be helping me a little--