I went through something similar last year when I just couldn’t perform anymore. The tasks were easy but I could not find the will to complete them. In my case I changed industries (went from Game Dev as a professional to Software Development) and as Trevor Ewen said in his response, found myself doing a whole lot of physical work.
In the end it all came down to stress for me, as the end result of three factors on your list: repetitive tasks, rushed deadlines and the poor diet.
If there’s anything that keeps me wanting to get back in the field is the last item: Stimulation! Say what you want about games these days but at least there is more Variety in them than accounting software number 6!
Great read, thank you!