Gotcha. At some point you have to consider cutting your losses. Controlling environment is key. The first step is to eliminate all the problem people who cause triggering. If you are with someone who has BPD and you intend on staying with them, you are going to have to cut out a lot of people and usually that has to start with the BPD person's family who caused the problem in the first place. It takes a few years of off gassing after that to be in a good place. I'd say about 3 years. Keep all narcissists away from your BPD person. They are triggering. If you are a BPD person's "safe person" you are going to get the blunt end of the triggering because they would rather not blow up on a stranger or acquaintance. I recommend this book: https://www.amazon.com/POWER-Surviving-Narcissistic-Collection-Narcissism/dp/1945796324 In dealing with narcissists, which are almost certainly the source of the BPD, start treating them like emotional retards... because they are. The lack of empathy makes them easy to deal with once you understand it. https://www.reddit.com/r/raisedbynarcissists/comments/7pje8n/neutralizing_the_poison/ Having the tools to deal with conflict is what BPD people need to start with. Also, in dealing with a BPD person, who will have inherited narc traits, you will have to use the same conflict resolution skills on them. This means your primary responsibility will be staying calm, cool and collected and not taking the emotional bait. BPD people NEED to be sober. If you are living with this person, you will have to practice sobriety, as well. Don't be under the influence of anything around them because it triggers them in ways that they won't even understand. Especially alcohol and cocaine.*ESPECIALLY ALCOHOL AND COCAINE.* Think of BPD as mild multiple personality disorder. You are going to have to find the triggers that keep the "real" person around. When in conflict you can, potentially, summon the real person back. Walking out and leaving until the real person returns is usually going to be your only option until you get your head around how it works. You will also have to identify the triggers that summon the bad version and avoid them like the plague. This will usually be abandonment (a double edged sword when you need to walk away from the conflict), sarcasm and criticism... even joking mild criticism. And, of course, alcohol and cocaine will almost always summon the wrong person. Without knowing many more details, I would say this is where you start, since it seems you are looking for answers. I studied neuroscience, so this area is along my line of expertise. I've also had to deal with rabid narcissists and have learned how to tell the difference between a true narc and someone who was just raised in a narc household who inherited narc traits.