Your post made perfect sense and, many of us have been exactly where you are. So, we completely understand your fears and your frustrations. Is there any type of job accommodation that your employer can offer you? Something less stressful, less physical, less wear and tear on your body? Perhaps you might think about having this discussion with your employer.
I had to finally give up my job due to the above three issues. However, I started my own business working out of my home. In this way, I take only as much work as I want, I work my own hours, and I do not have to call in sick (I just don't work). I know that this is not possible for everyone, but it is something that you might want to think about if maintaining regular employment continues to become too wearing on you.
I do wish you the very best and hope that you find some relief soon.
Peace and Blessings
Look For The Good and Praise It!