15 years of this is a lot more than many people could deal with, I guess you are a strong and capable person, however nobody is an inexhaustible well of care and endurance. Too much and one becomes ill oneself. There is a temptation not only to expect more than is possible of oneself but also have a (undeserved but strong) feeling of failure if things do not work out.
You wonder why you can have successes at work with those that have chronic conditions but cannot succeed with your husband. I'm sure you are aware it is a different sort of relationship and it may be your husband does not have enough motivation to do more.
Looking at myself I can see that it was basically my efforts - yes of course I had tons of help and support - that allowed me to improve. Seeking and accepting medical professionals that clicked, taking the meds despite side effects with the early ones, doing the exercises, trying to do more in the world, accepting responsibility for my partner's welfare and so on, I"m sure you know what I mean. I'm nothing special, there are crowds and crowds who have done what I have, probably better.
I had two driving things to spur me on, first the place I was in was so frightening, horrible and unpleasant I wanted out - and treatment was a way. Second I felt very guilty about the way I was and had been treating my partner - I did not want her hurt.
Looking at what you say about your husband I get the feeling he is coasting, not trying -my apologies if I've got that wrong. True he may have had employment and back problems, it does indeed have an effect, however if he is not trying then perhaps you might consider some ways to make him start.
I don't know how, you would have a much better idea - what do you think?
The other thing I'd like to mention is that when my wife (also a nurse) was having to cope with me - and work - and look after the offspring - and run the house etc she had her mum on hand. This made an enormous difference, both in terms of practical help, but maybe more importantly as moral and emotional support. Do you have anyone to support you?