Draining is the right word, for sure. It
is exhausting trying to get agreement/cooperation with a pwBPD who doesn't seem to be in treatment or working on improving.
Finding ways to remove yourself from the situation could be helpful. He's going to feel whatever he feels (and those feelings can change rapidly and wildly, depending not on what's going on externally but on his own inner experience) -- you aren't required to be present in the room as he uses unhealthy approaches to "managing" his feelings.
For example, here:
Then asks why I bought our son a new water bottle and I said because his other one was mouldy. Cue a rant about how no one washes the water bottles etc (not true).
I'm guessing it's pretty obvious when the "rant airplane" is taxiing down the runway and is ready to take off? You can observe changes in tone, words, body language, etc?
It's OK for you to exit the room when he wants to fly the rant airplane. No sense in both of you going on that journey. This protects you from hearing the rant and the blame.
...
In terms of this:
I mentioned to him that I think the cat needs to go to the vet (scratched eye getting worse) and his said I have to wait till pay day as we can’t afford it. I didn’t reply but got on my phone to look for appointments and he said ‘don’t give me that look, it’s not my fault’. I didn’t give him any look, it’s his default mode to automatically assume he is being blamed.
I've found it less stressful for myself to assume that my H's kids' mom won't cooperate at all -- not even at a basic level -- and that anything that needs doing, I need to figure out 100% on my own. Then, if she chooses to help, that's a bonus, and if she chooses not to help, then my plans aren't in turmoil.
The cat can't wait for your H to be in a cooperative mood. If I were in your shoes, I might be brainstorming ways to get the cat treated first, then sort out a payment plan directly between me and the vet. Again, he may have all kinds of emotional responses to that -- you can decide if you want to stay in the same room with him while you make an appointment, or if you'd like to be elsewhere (somewhere peaceful, like in the bathroom, in the car, at the library...) while you set stuff up.
It really sucks when pwBPD aren't cooperative.
Goodpal sums it up well:
Reminding yourself that this is his issue and not yours may be helpful. It's difficult to detach all of the time but it does help to maintain your sanity.