Am I experiencing burnout?

No. Not burn out, but rather your getting sick from grading.

You can’t force yourself to like what you don’t like.

Best to consider changes before things get worse.

I suggest asking a career counselor, in confidence, about your options?

You might want to read a book, or quick YouTube video on finding ones Ikigai?

A good job is in the sweet spots, between passion, pay, usefulness to society, and something that your good at.

Or barring that you can delegate the easy stuff to a student teacher or assistant.

I guess burnout is one way to term it. Like all repetitive actions we can do, it gets old quickly. Trying to push yourself through is sometimes worse than taking a break. When you feel yourself drifting, finding your attention wavering, take a break. Go exercise for 30 minutes or so. Work out, go for a walk or a run, do yoga…anything. Do something physical rather than mental. After that, when you return to the task, you will find you can focus again.