A mental health day is a justifiable sick day. Better to have a mental health day than to be sick for days because you wore yourself out. Look at the calendar and see when your longest stretches are without a break (no Monday off or other holiday). Make a note of possible days you would want to take off to give yourself a break and tentatively put them on your calendar. Maybe plan a long weekend or take a random Wednesday off (my favorite).
NO is a powerful word. Don't be afraid to use it. You can always change your mind and say yes later (on rare occasions), but it's really hard to go back on a yes.
Put a sign on your classroom door when you are working on planning or grading. You'll have less interruptions and have more focused work time.
Do not disturb your heart as well. Don't listen to negativity. Have a standard answer when...