In bipolar the self-destructive behaviors are because the individual feels all powerful and invincible, whereas in the BPD the self-destructive behaviors are an attempt to stop pain. It's a huge difference.
In bipolar disorder mania or hypomania must be present for the diagnoses to be made. Many individuals with the BPD have been erroneously diagnosed with bipolar because of the mood swings, even though they've never had mania or hypomania.
Bipolar I is the classic mania alternating with depression. Bipolar II is depression alternating with hypomania. Bipolar III is when the hypomania or mania occur only with antidepressant medication. Mixed bipolar is also called rapidly cycling bipolar where the person alternates between severe depression and severe mania in a very short period of time - sometimes minutes or hours. Cyclothymia is likely a form of bipolar with mini highs and mini lows in a cyclical manner.