You are doing well. We can't really change others, we can only change ourselves, our attitudes, how we view and handle things. Try to find and do things to bring happiness each day, even if only for a few minutes. This won't last forever, but treasure the moments whether with him or away from him. You do have some choices, so try to learn ways to work towards some freedoms. Try to give yourself breaks, it works wonders! Especially exercise outdoors like walking or biking.
What you are going through, is pretty typical caretaker stress. You are taking care of a seriously ill loved one. After you think about this awhile, try to make some decisions about some changes you need to make, you should sit down and talk to your husband about it. Explain that you are reaching burnout, and need to take action to make some changes so that you will be able to continue helping him effectively. If he is as nice as you say, then he should be all for it; it is in his best interest. It will involve leaving him for some periods of time (to walk, go out with a friend, or whatever) but that must be done on a regular basis to keep you healthy emotionally and physically. Give him a list of things to do while you are gone, that will help you and him.
We would all love hearing from you in a few months as you sort things out and make some small changes...