Find a quiet place to sit on the ground, close your eyes ANC try focussing on nothing but your breathing, in and out. When thoughts invade your mind, return immediately to counting your breaths in tens.
It does work!
DBT has been highly recommended.
Why can't you remember when you are not in session? Are you too upset to be able to remember? Are you able to remember it when you don't need it?
Ok, let me start by saying that I've not done or practiced DBT and I haven't got BPD (that I know of). Though the problem sounds the same as the one described in 'crucial conversations', whereby your brain has gone into fight, flight or drop (looking like you are dead) mode. This is a primitive reaction that everyone has that is quite essential for survival (but possibly not in the modern world).
What you need is a circuit breaker. Can you do some deep breathing, look at the page http://www.betterhealth.vic.gov.au/bhcv2/bhcarticles.nsf/pages/Breathing_to_reduce_stress?open, half way down under the heading Abdominal breathing. It may be good to practice for the 10-20 minutes, but when you feel like everything is too much, just start, even if you only do it for a few breaths, just long enough to exit fight,flight or drop mode.
Another alternative is to state that you need a little space at the moment, take yourself somewhere quiet and let yourself calm down, then go back and tackle the situation according to your training.
Good luck, and let me know how you go.