One of the ways to distinguish the two conditions is that people with bipolar can function pretty normally in between mood episodes, whereas borderline personality disorder generally involves chronic mood instability. But with rapid cycling of Bipolar... you also suffer from chonic mood instability.

I just read this and thought I would share it with you:

“Another complicating factor is that bipolar disorder and borderline personality disorder often occur at the same time. In fact, almost 20 percent of people with borderline personality disorder also have bipolar disorder, and a similar percentage of people with bipolar disorder may also have borderline personality disorder!”

My old pdoc use to say that the major difference was trauma, that people with borderline have a lot of unresolved childhood trauma, I have no idea how true that is.

My personal story: I was dx with borderline first, then with bipolar I... my current pdoc doesn't care much for labels, he just wants to find something to work for me. I know with borderline, you don't get much symptom relief from meds, cognitive, psychotherapy tends to work best. I also believe I am in the 20% because I fit really into borderline but have cycling and crazy mania.