Rumination without a solution leads to depression
And there is almost always no solution to borderline and/or narcissistic abuse. So try your best to internalize that you can't change the outcome logically because disorders don't really have a grounded reality in the first place.
Edit: why,: rumination just makes the negative neural pathways stronger and takes away from your capacity to focus on positive experiences. This develops into learnt helplessness outlook towards the world and your brain is consistently full of cortisol. As a result your energy for executive function drops.
You can combat this by doing a bunch of heavy physical activities that force motor control. You start with small 3 to 5 minute tasks (it doesn't take long if you're in a bedrotting mood) and build it up over the months. You must do it to the point that you feel EXHAUSTED. You essentially want to starve your own brain of energy from the rumination.
Actively you can give your brain say 3 minute time limit to think about the fucked up pwBPD experience and then get up and stop. Though I had more success with physical activity.
This must be in addition to therapy from a narcissistic abuse specialist. Don't go down the gym rat route in an attempt to distract yourself, it won't work in isolation without say CBT remodelling your thought processes.