Help explaining to gf about bpd and family

I dont know how to go around exlaining to my girlfriend about my bpd i know its not an excuse for my behaviors or actions but theres time where my family or friend or gf will be like why do u have to be so depressed all the time and im just having a hard time explaining it to my gf as far as my family i dont think they even know about it and i dont want them to but yeah helping me explain it to my gf would be great thanks

Hi, GDhack! You could ask your girlfriend if she has ever been sick and if she could really help how it felt. When you have a bad cold or some kind of physical pain, you'd find it strange to have to explain to people that you can't control what's happening to you or that it's not your fault that the illness is there. You could tell her it's just how your brain is wired, even though your mood swings might sometimes have to do with stuff that happens in your day to day life and that the illness is pretty much about how your brain reacts to your environment in a way that isn't exactly "normal".
Also, she might be feeling helpless when she sees you're in pain, so you could tell her that her support and her trying to understand would mean a lot to you.