If you run a search for this topic, I assure you quite a few threads will pop up, its a popular topic of discussion, on this board and the BPD one.

I've managed to untangle the BP from the BPD, and it has helped immensely in calming things down.
Unfortunately I cant get into it now, but remember these main things:

BP is organic, BPD is inorganic... It's a mixture of messed up perceptions, skewed thought patterns, and unhealthy behaviour.

BPD is reactive, its dependent on relationships, people and environment. It's an interpersonal issue, with interpersonal triggers. Eg, my boss may make me want to hurt myself, but before I can react my bf shows up with a bunch of roses and hugs...
The focal point of my mood will automatically shift and become my bf, which means my mood will change.

BP is random, besides very specific triggers like stress and sleep deprivation, nobody causes a BP episode / moodswing.
Randomness is actually part of the dx criterion, last time I heard anyway.
Also. Main thing, Once in a specific mood state, something or someone won't just snap you out of it. It's over when its over.

BPD is destructive, but pliable with the right tools. Meds shouldn't be an issue because they rx the same stuff for both dxs.
I've done DBT for two years now, it's been immensely helpful, and yes I do it in a group, it's better that way, because we get to see similarities in our behaviour, discuss where we go wrong and how to behave better.