I can't tell you the difference from a personal perspective but as a support group participant I have seen the mood swings of people who are diagnosed with both bipolar and borderline personality disorder in action during support group meetings and they are instant reactions to something someone says or does even when not directed at them or when relating an upsetting event that occurred to them. They can go from rage to abject misery in seconds then be ok again all in the space of 90 minutes. I think just like its hard to explain bipolar to a person who doesn't have it, explaining bpd is also hard to do.