Apologies for the late response to this.
Livednlearned, thank you so much for your kind and thoughtful response. I've been implementing some of your suggestions.
The author/researcher Brene Brown talks about underfunctioners and overfunctioners.
I read up on this concept and wow, I see it everywhere in my life now. I've been trying to more consciously underfunction where I can, but there are consequences to my underfunctioning - some that I'm ok with, but many that I'm not. One that I'm ok with temporarily is that my house is messier than ever, but soon I won't be ok with this because 13yo's caseworker will be coming by. Last week I took myself off homework support duty while 13yo was still doing learn-from-home;13yo submitted no assignments for the whole week and now has even more catching up to do. Luckily school went back to in-person this past week so there's a bit less pressure there.
I talked to my partner about how I needed him to be more present, take on more responsibility, and spend less time on computer games and other hobbies and more time interacting with 13yo to take some of the pressure off me. He said absolutely, he could do that. Yesterday he spent 10 hours playing computer games, and also refused to eat until 8pm (a common occurrence when he's stressed - food is something he can control) so he was hangry and nasty all day, while I got groceries, made sure homework happened, and baked cookies with 13 year old. Today he's all lovey, said if I wanted him to take over homework duty he could. I let him know 13yo was taking a 10 minute break, and if she wasn't working again in 10 minutes to remind her, and then I left the room to go relax. Came back an hour later, 13yo was still laying on the couch having not restarted homework, and partner was playing computer games.
Unfortunately, waiting for partner to pick up the slack in a lot of areas means risking 13yo's wellbeing and placement with us (I know most kids her age shouldn't need supervision to complete homework but unfortunately with her issues she needs a coach in the room most of the time.) There is something unrelated I've decided I'm not doing though - we're supposed to get married in the fall and partner wanted the reception in our home/yard. I told him today that I don't want the responsibility of all the extra cleaning/planning/organizing that will entail. Instead of taking it upon himself to share in that work, partner agreed that we could have the reception elsewhere.
Another win is that I'm no longer going on grocery trips where we take my BPD MIL. The last time partner got too angry and left me alone to deal with her accusations. I can't go fully-NC with her because we're fostering her kid, but I'm now only communicating with her via text or email and only once a week. We were taking her for groceries out of kindness, but I don't need to hear her angry tirades on top of everything else. I already deal with her disorder every day in its effects on our family.
And on another positive note, to everyone's amazement, my grandfather is recovering.