Here is something I just wrote in another reply, so I feel I can steal it from myself:

"Sometimes things that happen or things that people say remind us of past events that were very stressful or frightening to us, and we cannot really tell the difference between a present danger and a past one. There are reasons why a brain might need to identify a possible danger very rapidly, and it relates what is happening now to what happened before in order to identify the present event. But that reaction also has dangers, because it can be mistaken."

Maybe being a borderline means that you have had experiences that make you unstable when triggered by present events, and you feel that you have to react instantly in order to cope with what you think is happening -- only you can make a mistake and the thing that is happening now is not the same (and you are not the same) as the thing that happened before.

I think people can understand this if it is explained to them. Until the next time that you get triggered, when you will have to explain it again!