Hi OffRoad!
Would telling him I am very sorry I didn’t understand how scary it was for him and if I had it to do over, I’d make sure he knew how worried I was about him help at all?
That is wonderful. It is called a repair, and every relationship needs them. It helps if both partners are able to do this, but in the case of an RS with pwBPD, it may just be you making the repair attempt. I do this with my wife, and it actually can be effective at stemming the flow of a dysregulation. And it's because you really mean it. It won't always stop them from being upset, but my gosh, it's coming from your heart and you really would do it differently in hindsight if you could. That is a hard thing to refute, even for pwBPD.
I hope this goes okay for you.