9 hours a day is a LOT! Do you have breaks in between?
I find doing 5 minutes yoga stretching or HIIT helps. Doing stretches that counteract being hunched over a screen is vital.
Having a coffee break at the kitchen door, even if it's raining, just to feel air on my face and watch the birds on the feeder.
Take a proper lunch break and go for a 30 minute walk. I try to do this most days.
Make sure you have really nice, easy to cook healthy food in for dinner.
Try to avoid too much screen time in the evenings. I do slump in front of TV a lot but feel far better if I go to bed with a good book, or wrap up and take a night walk to look at the stars.
Have 'office hours' once a week where you deal with student/parent emails. At other times send out a generic reply that Truly is currently in a teaching session and will reply to your email on Saturday morning.
Make sure you plan fun stuff for at least half a day each weekend - something to look forward to, something you have done which breaks up the monotony.
Staring at screens is so tiring, teaching online is draining, and my neck is so sore now despite all the stretching. And I do half the hours you do!