Ive always thought its a daft system when a company relies on one person....what if you were ill?...I'll correct that and say that stress is an illness .....I hope the changes you talked about earlier in this thread make a big difference for you.
Has your boss got a point re Xmas day? If so then...
Recognising it as a you have done is your first step...all the best!
PS the exercise thing always helped me...walks (some where you can interact with people in a non work way)....watching the tv whilst on an exercise bike at home...etc