Gosh this person seriously sounds abusive. Any way you can maybe try and go for a bathroom break or disappear early? Just to limit contact with this person as much as possible. I'm getting the sense work can't move this person on and they're clearly not receptive to change either.

The perception of having lost or given up doesn't matter as much as your ongoing mental health, you need to be able to power through the remainder of the year and be well enough for next year, and if that means getting a Dr to just write up a mental health sickie for 3 days (they never need to put the reason why on the cert) I would personally go for it. Life is too short and precious to be worried about perception IMO when you're as seriously affected as you've been describing