I was in the same boat at my first startup (a series of 70-100 hour weeks and I was completely fried by year 2.)

I had accumulated (and untouched) vacation time, so I wrapped up anything critical, drove to SFO, got on a plane to Mexico, and stayed for two weeks. I did not bring my laptop. I did nothing but swim, go to the beach, read (physical) novels and have nice meals at restaurants.

Exercise and sunshine pulled me out of a truly devastating spiral; it may not work in the same way for you, but could it hurt?

Good luck: I really feel for you.