OP, I don't know a solution to this, but I'm curious if you've tried intensive exercise routines, low calorie diets, time in nature, cold showers, socializing and, working on your non-work life as a whole? I've seen this help me out in similar battles and I have also seen them recommended to others.
I also have a more general question to everyone on this thread, what is the difference between burnout and demotivation? How can one tell the difference?