moreaware, and welcome!
First off, I want to say that you ARE doing it right, and you just need to stick with it and be firm about it.
Boundary enforcement is by its very nature a HARD thing.
You set your limit.
If behavior is less than the limit, you ignore it / put up with it / etc.
Once behavior crosses the limit, you immediately take action to protect yourself from the behavior.
It isn't particularly mean when you do it. However, there is nothing "nice" about it either.
When you do it this way, you are doing it right.
I will first tell him I will remove myself (go into another room), if he continues. However he often does continue which means that I have to take things a stage further, by telling him calmly that I will leave the house if he continues.
One tip: warning him that you will remove yourself (from the room or the house) isn't required. All that is required is the action of going away.
I'd suggest saying something like "I won't be spoken to that way." and leave.
When you leave the house, here is an OPTIONAL bonus thing you can do: Say that you will be back in a specific time period, like 20 minutes or 2 hours, or even in the morning or after work tomorrow, depending on the circumstances. If you do this, come back on time, but be prepared to leave again if he's still being abusive. (You didn't promise to come back and be an emotional punching bag. You promised to come back!) This can help because pwBPD tend to have a strong fear of abandonment which is triggered when you leave. If they can trust you to come back when you say you will, it will help reduce this fear.
it is impossible to have a life where I keep having to leave my own house, so I find it incredibly difficult to keep my boudaries consistent.
No, you don't want to live the rest of your life this way. One way or another, you won't have to.
First off, if you are inconsistent on enforcing your boundaries, you are really shooting yourself in the foot. What this gives him is called "intermittent reinforcement", which is the same thing that slot machines do, and humans are wired to be very strongly conditioned by this sort of thing. You keep pulling on the one-armed bandit because it will *sometimes* give you a jackpot.
In addition, be prepared for a major escalation on his part. When you do start enforcing a boundary consistently, the typical reaction is called an extinction burst. There's a video of what a normal person does when they go into an elevator and push the button and nothing happens. They push it again. And again. Then have a little fit where they smack the button a dozen times in quick succession. And finally after a bit of cursing, walk out and take the stairs.
So... .expect to have to live through that. And once you do, he will start to believe your ACTIONS, that you won't accept verbal abuse, etc. from him. It really isn't much fun to yell at somebody after they've left, and doesn't give him the release he's used to getting by taking his crappy emotions out on you.
Once you get your boundary enforcement rock solid, and he realizes that it is that solid, and he cannot get around it, he will stop trying so much. Or will respect to your first level response. In a bit you probably won't need to leave the house anymore, or at least not very often.
