I have both BPD and HPD............. and i think the difference would lie in the stableness of the sense of self.
BPD's can often seemingly change from one personality to another, depending on the situation they are in. (mentally, environmentally etc.)
So, for example.........if i'm with some angry folks......i tend to become more angry.
If i'm with some happy folks, i tend to become more happy.
If i'm with a bunch of kids.........i tend to act like a kid.
If i'm with a bunch of old ladies...........i tend to talk about the good ol' days. And how young people have no respect anymore. (ok that isn't true....but you get the point)
A person with BPD, may have a lot of identity issues.
Like for example, i sometimes see myself as a nice caring person. Other times, a very cold calculating selfish person.
I often drift back and forth between liking people and feeling nothing for them at all. Even people, i currently (HPD) value.
Also, my opinions...........can rapidly change.
"i'm not into guys at all" "ooh he's hot, i'd do him"
Some days i log on here and actually want to help people and care about how they feel.
Other days, i log on here and just want attention and am as far from caring about how anyone feels as i could be.
(more like some hours, some minutes)
I think most HPDs, generally know what they want.........they just stop wanting it.
Whereas, BPDs, constantly shift between wanting what they want, and then not knowing if they actually want it.
Also, HPDs don't seem to be as outwardly sad. Plus i guess the emptiness.
I'm not sure if HPDs get the whole emptiness thing.