Get your doctor to write that you need some time off. Take a medical leave for as long as you can get it. And start looking for another job.
Taking medical leave accomplishes two purposes. First, it gets you out of there for a while. And when you go back, everytime your health starts deteriorating again, take another leave for as long as you can get it.
The second thing that accomplishes is that your principal cannot fire you because then you can say it's because of your health and that's highly illegal!!
I had severe migraines for a couple of years and my doctor was wonderful about writing a formal letter saying I needed a two or three week leave. This happened several times during those two years and I finally got a job in another county. By the way, your current employer cannot give a bad recommendation based on health issues.