Hi Larmoyant,
Splitting (or black and white thinking) is a typical defense mechanism used by someone with BPD to cope with their abandonment trauma. It appears that the closer we get to the pwBPD, the more likely it is that we will end up representing the original person with whom the pwBPD experienced their abandonment trauma (e.g. a parent when they were a child). We become the trigger for all that pain and the best way many people with BPD know how to deal is to split us into "all bad" or "all good." It's an immature way of coping, but it's the best a pwBPD can do in the moment, especially in the throes of dysregulation.
I understand your confusion. It happened to me in my relationship and it felt devastating to suddenly be cast in the role of a mother who so deeply hurt pwBPD. PwBPD recovered quickly from that dysregulation, and understood it as such, but I knew it would happen again and again, and it was too painful for both of us.
ACOA (Adult Children of Alcoholics) mentions "all or nothing" thinking as a coping mechanism. It is also a feature of people who are depressed. So, it's not exclusive to BPD, we all are susceptible.
Here is a very enlightening thread about
Splitting What do you think?
heartandwhole