I've only skimmed the thread, so apologies if I have missed anything.
OP is clearly suffering and I feel for her. However, I do think there is a danger that this is becoming (or is) another "let's complain about people on PIP" thread. And I'd also argue that people who are unable to work due to mental illness are, on average, far less functional than people who can hold down a job.
For reference, I am currently not claiming PIP, although I have been advised that I could because my mental health is so poor. I think part of why I am not claiming is a.) I cannot bear admin and b.) I am ashamed to be where I am.
Okay, soapbox rant incoming.
Up until fairly recently, I was a very high-functioning mess. I worked, studied, socialised, had caring responsibilities etc. despite having diagnosed anxiety and depression. I have had panic disorder since my teens that is largely treatment-resistant. I probably qualified as bulimic. Just a huge, huge mess. I received some CBT and bounced between antidepressants, but basically never got better.
One day I just... snapped. I was at work one day and ended up hiding in the stationery cupboard. I really thought I was going to die. Developed paranoia about various things, including the belief that I was allergic to everything I ate, was being poisoned, or was going to drop dead. Cue nervous breakdown. Became an agoraphobic wreck, which I still am. I'm only 25. This really isn't a lifestyle choice. It's absolute hell. Every trip outside the house triggers a full-blown panic attack. I know my mental state is harming people around me, including my mother, who badly needs my support. I really miss having reliable income, seeing friends etc. But I can't go anywhere. For a while, I would try to go to job interviews etc. and would get dressed, get my bag and just end up... sitting, unable to move. I did manage to do temp work for a few weeks, which involved having to sneak into the toilets to cry and hyperventilate and throw up because I was so terrified.
I think what I am trying to express is that all it takes is for one really bad day to send you from "miserable but functional" to completely non-functional. For that reason, I really can't blame people for needing time off work due to mental illness. It's not a choice. I would never, ever choose this for myself.