Trust me, this is a true blessing. I was in a very, very similar situation just earlier this year. At least you were written up. I was called into managment and was told "we have concerns about you being happy". When I asked what brought said concerns I was told "We aren't going to discuss specifics". Talk about bullshit. This happened twice before I finally started pushing back some which I knew was just writing my eventual exit.
I was working a shitload of hours, I was stressed to no end and there were no signs of it getting better. The day we agreed that I should part ways I was down about not having a job. It took all of 2 days before I realized the stress of not having a job wasn't half as stressfull as it was working at that place.