Hi Crispy, I'm sorry to see how much of a struggle life is. I do t want to be that person that suggests something you've probably already looked in to, but I have B12 Deficiency and your symptoms are very similar to it.
Unfortunately with B12 Deficiency, the regular serum B12 test alone doesn't always tell the whole story (as in your numbers can be well within range for your blood serum, but we can't measure what's being utilised by your cells and more along those lines). If you're interested in more information regarding this, I'll get some links to YT videos, websites etc that give more information.
I also have other conditions besides the B12 Deficiency but after 5 months of every other day injections (overseen by Drs but done myself), I am feeling more clarity of thought for periods in the day. The rest of the time feels like my brain has concrete poured over it and I have to go to bed and sleep.
It can have a massive impact on mental health and this was very obvious when my dogs had deficiency too.
https://b12d.org/psychiatric-psychological-symptoms/Importantly, if you think it could be low B12, please don't start supplementing before getting tested as that can skew results. Let me know if you'd like more info (I'm more than happy to share if you're interested but it takes a lot of mental energy to find the stuff and if you're not interested, then that energy can be used elsewhere (pacing)).
Take care xx