I've been asked by at least one temporary placement agency to supply a letter from my doctor saying that I'm fit to work

There is a purely practical reason behind this. If your doctor thinks you are not fit to work, and you work somewhere even without the employer knowing this, the employer's insurance is negated. If you or anyone else get into an accident at work, they're wholly liable. So it's to do with making sure someone else pays for it if something goes wrong (with you or anyone else), and I have no clue why they can't just tell people that. As though it's more fun to make you feel singled out and humiliated.

WRT to the rest of it: I've been there - hell, I am there. There's a lot I could say that won't end up meaning anything to you, because we're different people and similar experiences don't mean similar solutions, yada yada yada.

The one observation I want to make is this: the desire to have another person around to bully you out of bed and make sure you take your medication looks like you want to delegate responsibility for that onto them, perhaps so you don't have to feel so guilty about not doing it. Do you think that's a fair comment, or am I totally off the mark?