I feel for you mate. I worked at a v large High School as Network Manager, but with no support except for a few of the pupils we recruited. I was working 9+hours a day, plus nearly 2 hours travelling. Same prolblems with sleep, diet, stress and finally quit, but not in time to save my marriage. Had nearly 3 years of not working (with associated stress of not being able to pay the bills etc). Now working for a great company. Not a lot of pressure but enough to make the job interesting.
Oh, by the way, it's not all over at 50. I hit that milestone a couple of months ago!! Good luck..