Just my thoughts. While my spouse has learned coping skills for the present time dysregulation, I've not seen him be able to apply that retroactively. So, when something comes up from long ago or triggers the same emotion he felt in a long past scenario he sometimes responds with alllllll the unaddressed with emotions from whatever long ago scenario. For me this isn't so much directed at me as dredging up our past issues, as it is that a disagreement with me might trigger those emotional memories and cause dysregulation. BPDh has some pretty serious PTSD from childhood and if I can still get through and he can identify the emotion we can agree to just deal with the emotion and situation from right now, and pack the rest back up to deal with at some later time. For example one episode he exclaimed that he felt powerless and I realized that it was triggering the emotion he felt as a child seeing his mother being abused. I was able to validate that he was being triggered this way and he visibly calmed down and was able to better work through just the actual situation of that day.
I know I've brought up things from the past in the heat of the moment, and I would have not done that if I had a cooling off period before discussing it. I'm thinking that a boundary might be that we need to stick to the current situation for the conversation to be productive and if the person still feels the need to discuss the past issue, we can make a time to address it. Like a 24 hour waiting period.