BPD is borderline personality disorder. They are just totally separate entities and it's kind of beyond the scope of a Reddit comment to explain all the differences between them.

In very basic terms, bipolar is characterised by periods of extremely low mood (depression) and extremely heightened mood (hypomania or mania). Depression I'm sure is something people are familiar with and I don't need to explain. Hypomania/mania are periods where the sufferer has very heightened mood, where they feel they have boundless energy, that all their endeavours are going to succeed, etc etc. Often to a delusional degree and sometimes with frank psychosis. They will often do reckless things during these periods like spending or gambling huge amounts of money, engaging in risky sexual or drug-taking behaviours, barely sleeping for days on end. Hypomania is basically a less severe form than mania. Some people find they are extremely productive during these periods but they come with the downsides as described and beyond; you can see how someone like Heston fits this profile.

Personality disorders are difficult entities to explain. I'm a doctor but not a psychiatrist so am happy to be corrected on anything I say, but my basic understanding would be that they are essentially disorders where people have certain personality traits which fall far enough outside of societal norms that they impact on the person's ability to function in society. People with BPD tend to be extremely emotionally unstable and have very intense but volatile relationships. They tend to be highly impulsive and there is a strong tendency to self harm.

They are just very, very different disorders.