Like BPD, a person with bipolar disorder experiences extreme shifts in the mood but between depressed states & episodes of mania, they may experience periods of a stable mood. Bipolar disorder begins with the brain structure itself, genetics, and family history. It is also mostly treated with medication, often lithium, which helps stabilize mood. Psychotherapy is also helpful to aid the person in managing complications in his/her life.
When a person with bipolar disorder is not in a manic or depressive state, they demonstrate the stability that the borderline personality does not show. When the bipolar person is in between episodes, they function pretty well. They can have normal relationships that might be disturbed by their periods of illness, but when they are not experiencing episodes, they have stability that you do not see in the borderline person.
Bipolar disorder is more fixed in the biology of the complex nervous system and is much more responsive to medication. Borderline personality disorders strongly involve the psychological level of the mind in addition to the biology of the brain & nervous system. Mood swings of bipolar disorder are more random & less related to events, unlike those of borderline personality.
Both illnesses can be successfully treated. Too often, those with borderline personality disorder are treated for depression or bipolar, when it's more complex than that. It is essential for a person with borderline personality disorder to see a specialist and get proper treatment.