Borderline Personality Disorder is, as the name states, a Personality Disorder, which should not be confused with biological disorders such as Depression, ADHD, Schizophrenia, or any other such disorders that deal with brain chemistry. It has nothing to do with one's brain chemistry, and is speculated to be more of a learned form of behavior, though the exact causes are still unknown.
I am in the midst of earning my Master's in Psychology, (will then earn my PhD), and despite how I have met and even worked with many people who think that they might have BPD based off of some some of the common characteristics that are often listed on the Internet, it is actually quite rare and VERY extreme.
I have honestly only ever encountered one person who ACTUALLY full-blown BPD: My father's current (and hopefully soon-to-be ex) girlfriend. I am sad to say, that she is easily the most horrible person that I, and many others even including my father, have ever met. Her actions and how she reacts to many things seem unreal--as if she is a character in a book or movie as opposed to a real person, because it is hard to fathom any actual person acting that way. Some examples of her behavior include:
• Impulsively divorcing her husband and then being surprised that it made her life very different.
• Criminalizing ANYONE of her old friends who still remains friends with her ex-husband, and assuming that they are all being friendly to him ONLY with the intention of "hurting her".
• Somehow convincing my dad to allow her to move in with him only a month after she started dating him. She moved out a few months later, but only after she BOUGHT a big house that she cannot afford and has trouble keeping up, as opposed to renting a place.
• Lying to her children about her relationship with my dad, and making a big deal about "sneaking around" to see him. Such as, on a weekend where she had her children with her, (her ex-husband has primary custody), she randomly showed at my dad's house, as opposed to being with her children, and then made a huge fuss about how she was worried her children might find out she was there.
• Taking Christmas gifts from a charity meant for extremely impoverished families because she wanted to show her children that she did not have as much money as she did prior to the divorce, as a way to attempt to turn the kids against their father.
• Constantly finding ways to bring up subjects that might cause my sister and I to get into fights with our dad whenever we visit, so that way she can isolate him. (She had to do a lot of digging to find those subjects, but she did, which is extremely creepy). Thankfully, my dad caught on to that.
• Her "fear of abandonment", which is a big characteristic of BPD, is NOT a simple insecurity. When my dad got a new job several months ago that caused him to have to move an hour away from him, she got extremely upset and tried to convince him not to take the job. When he went across the country to visit my sister, (who he only gets to see a few times a year), she constantly bombarded him with angry texts about how he was selfish to do that because it meant she had to spend a few days alone.
....These are just some examples. I could go on for pages.
However, that does not mean that ALL people with BPD are just straight-up awful people. There are also ways to get help, but the person with BPD has to be willing to undergo a lot of work to do so, as it requires more-or-less complete reprogramming and they would have to meet with a specialized therapist several times a week.