My theory is that suffering his first real setback in the East, and his inability to wrap up the West with Britain still in the war, not to mention the invasion of Italy, dealt his already narcissistic personality an embarrassment he was ill equipped to cope with. He was already in the clutches of addiction to numerous concoctions his quack of a personal doctor had been treating him with.
Look at a before and after photo of any U.S. President to see what stress does. Then imagine what Hitler put himself through.