Hi BB,
Does your employer have responsibilities to provide you with accommodations under the Americans with Disabilities Act? (Forgive my ignorance, I'm Canadian.)
For example, wouldn't they, at least, have to provide a TA to do your marking?
Are you tenured? Perhaps if they realize how much it will cost them to provide the supports necessary for you to do your job, they'll either back off, and give you more time to recover, or try to buy you out with a settlement.
I agree with the others about documenting your limitations through a neuro-psych, etc... Sadly, you might find that you will need to fight for your rights. As much as I hate them, you might need a lawyer to smarten up your employer.
Hang in there.