I'm deliberately leaving the explanation blank, because I've learned from experience that such explanations are often just something you make up after the fact.
What I can tell you is that I consistently reach a point where I simply can't find the will power to continue. I can only find the will to do something else, to go somewhere else. The underlying reason for a feeling is always difficult to pinpoint.
My feelings when I'm in the situation are a mix of stress, anger and hopelessness. I want anything but to perform the task that has been assigned to me. I want to get the hell out of there.
Whenever I attempt to address the problem with the people around me, their proposed solutions are always very plain: Take a walk. Take a vacation. Get some rest. Don't worry so much.
To some extent, taking a very long break does resolve it, but I can't take 3-6 months off from work every year. I have considered working half-time, but this leads me to ask why I want to work half-time in the first place? Why do I want to work less?
Possible answers:
1. I am doing something wrong.
2. I'm a fundamentally broken person who can never hold a job.
Option 2 isn't exactly what you call a constructive or positive attitude, so I'm betting on option 1.
Yes, if you are getting sick because of work then yes it's the wrong job for you.
What do you do now? Look at what you like and find a job that has many of the characteristics you like. You won't find the perfect job at the start but you can slowly evaluate the new job and move towards what you like. Volunteer if you can. Find a career coach to help you.