From what I've read and heard BPD and Bipolar are often mistaken with each other. However - I also read that BPD often coexists with other mental disorders and illnesses. With bipolar as well. Hence it's possible to have both BPD and Bipolar.
However - the difference between standard bipolar and BPD is (at the first glance) the speed of mood cycling. Once I've asked a person with Bipolar about how long her "moods" lasted. She told me that maniac episodes - a few weeks, depressive ones even some months. As a pwBPD I have my mood changes lasting hours or even minutes. Also - she told me that since she was taking meds (mood stabilizers), the symptoms intensity decreased and she was feeling fine. I've never heard of pwBPD that was ok just because of taking mood stabilizers, that seems to be only an additional, not main, help in BPD treatment.
And I've never experienced real mania - only the maniac-like states, again lasting no more than a few hours.
But then - there's also bipolar II and cyclothymia. I don't really know what's the difference between bipolar II and major depressive disorder or between BPD mood changes and cyclothymia.
Additionally - what can be misleading is the fact that some of the borderline people suffer from depressive states from time to time (as me). So how to recognize between BPD with depressive states, BPD with MDD, MMD from BPII and what more combination you're able to imagine. I don't really know.
Yet - I've heard something like BP is more "independent" and more strongly attached to genetics and neurology, whereas BPD is rather a result of bringing up and is much more "dependent" (like your mood is associated with environment influence).
But again - they still are not sure what causes BPD and I sometimes feel "bad" (scared, angry, whatever - any negative emotion) out of nothing.
I don't know if it helps, but I like this blog, the author seems to be quite objective and what's more - she has a sister with BP (being pwBPD herself), so the comparisons can somehow explain you a few things (I didn't read all the posts about BP, so I'm not sure if they are alright).
http://www.downwardspiralintothevortex. ... 0vs.%20BPDHave you ever experienced hypomania?