Two of my favorite articles for any time I feel like [it sounds like] you're feeling:
"You are Not Trapped Inside Your Life" - http://bonzaiaphrodite.com/2009/08/you-are-not-trapped-inside-your-life/
"You Are Flawed. And So Are Your Heroes" - https://www.nerdfitness.com/blog/you-are-flawed-and-so-are-your-heroes/
The words "horrible" and "stress" come up a LOT in your posts! It may be helpful to do some goal-planning - identify the roots of your stress, and what makes you feel horrible, then identify the solution/goal, and create sub-goals and mini-sub-goals in order to reach there. If you have an end-point (even if it needs adjusted along the way! even if it's far away!) you have something to look forward to.
For instance, if I sit and think about my financial debt, I can feel horrible and stressed (e.g. "it's my fault I got into this debt, it's weighing me down, it's affecting my credit score, it wasn't 'necessary' or 'helpful' debt like college loans, why can't I catch up and pay it off?"). But at the turn of the new year, I sat down, pulled up all my accounts and my budget, and created a payoff plan. It's going to take me two years, sure, but the goal I can keep at the forefront of my mind is that hey, in October 2018, if I keep with it, I'll be debt free!
So: it sounds like your job is an obvious and major source of stress. Clearly, quitting immediately doesn't seem like an option. BUT: think about what you'd like to be doing, and where it would make you happy to work. Does that position require additional education, training or skills that you don't currently have? Do you need to start networking with people in those positions to learn more about the positions and figure out when and where there might be openings, and what kind of people they're looking for? Set long-term goals, and then sub goals and mini-sub-goals to reach each step of the way.
Think about why people pursue education - it's not because school is fun (although that's often a nice plus!) or easy or pays well (or pays at all, or actually puts you in debt), it's because 4-5 years down the road they want to be doing something different than they're doing now. 4-5 years (or even 1-2 years) to pursue your goals can seem like a long time, but A. it flies by, especially when you create short-term goals and deadlines and you're working towards something, and B. when you think about your life in the long-term, do you really want to be doing what you're doing now forever, with policies that you're morally against, that physically makes your miserable in so many ways, or do you want to take some time, pursue your goals, and do something you really believe in, that fulfills you? Indeed for you, beyond fulfillment, switching careers will likely optimize your physical, personal, mental and emotional health and wellness.
BONUS: One more article on food that I love to read through when I'm struggling with food choices, or coming off a bad week or month of not eating healthy or right for my body: https://sarahfragoso.com/eat-food-that-doesnt-hurt/ - Sarah's blog is awesome, and she has a ton of great posts, plus my all-time favorite, no fail crockpot pulled pork recipe: https://sarahfragoso.com/beyond-easy-pulled-pork/ - this is one of those recipes I can eat for breakfast, lunch and dinner.
FINALLY: mini-sub-goal #1 - get a new work chair. Sitting in that chair is physically hurting you! Request a new chair from administration / save up for a new chair and buy it and bring it into work yourself / get a yoga ball to sit on instead / get adjustments for your current chair like a pad or back support or something. I literally just searched on Amazon for "chair back support" and got over 31,000 results. One or more of these solutions has to be doable. Again, per Bonzai Aphrodite, "you are not trapped inside your life."