From my experience, there are very clear distinctions in identifying bipolar vs BPD traits
Bipolar
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Moods last longer - days, weeks
Will become manic and temporarily alter their behavior in an almost obsessive way (such as cleaning)
Usually seek a psychiatrist for meds to stabilize their mood swings, will admit their deficiency
BPD - I know it's cliche but bipolar dont exhibit these key behaviors below
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Shorter mood swings - minutes
Idealization - they come on stronger
Mirroring - they pickup your habits/favorites to make you feel special
Devaluation - they think less of you over time, not as special
Triangulation
(read definition) - find alternative relationships
Push/pull - tests of your loyalty/devotion... . insecurity!
Tend to live in denial about their disorder and many times go undiagnosed because they avoid medical/professional help/intervention
Bipolar is also due to a chemical imbalance in the brain while BPD is due to environmental factors.
Some people are diagnosed with both BPD and Bipolar.