OP you poor thing! 💐 Can you discuss how you’re feeling with a manager, see if there’s another way? I have left two jobs (with no other job to go to) for the sake of my mental health and while I don’t regret it for a moment, there was no other way out on those occasions. There have been some really rough patches in my current job but I finally work for a proper company with decent managers where I am valued, so despite the occasional tears (literally!) I’m still here and don’t want to leave at present.
The first was a job I’d been at four years, and I was very happy there until management changed hands about ten months before I left. It was a small family business and the awful new manager put themselves in the position of HR also, so I had nowhere to turn - with hindsight I was absolutely managed out of that job.
The second was a job I took because I just needed a job, and they were offering the best money. The atmosphere was utterly depressing and it had been mis-sold to me in various ways, I started having panic attacks on the way to work and thinking how it might be nice if I was hit by a car, just enough for a week or two off work but with no real damage, of course. Realised the insanity of that and left after five months.
To be honest I think if you do it the main consideration is money/savings. I’ve always been a saver and have a DH so I had enough to fall back on and a safety net (we’re not high earners but can just scrape through on one wage if necessary) - but at the same time I believe you cannot put a price on your health and happiness, and no job is ever worth sacrificing those for. Bear in mind you might want to take some time out before even looking for a new job to rebalance - you don’t want to be as stressed job hunting as you were working! So budget for that too.
If you do quit, you do not need to put any reason for leaving in your resignation letter. They don’t own you, you can leave a job any time and it’s none of their business why.
When asked about it in interviews I simply said something along the lines of it no longer being the right fit for me and leaving without another job lined up because I could. It’s not held me back.