Eccentrica - I feel for you. I have had exactly the same thoughts recently - have weight problems partly due to underactive thyroid, (and partly due to complete lack of exercise!) and recently saw the dr after a humongous headache only to find that my blood pressure is quite high.
I have spent this last week having a really good think about everything. If you can do the same perhaps it would help you - I have just come to the realisation that I need to look after myself as well as everyone else. Sometimes that's easy to say but harder to actually do, because children and husbands and just being in the role of carer can become so ingrained in us - and it doesn't sound like your husband has been much help
However, the conclusion that's really helped me is you don't need people's support and 'permission' to look after yourself. It's ok to assert yourself, and to make a fuss, and even to lose your temper and stamp your feet about stuff. If you have to. It won't mean your family love you less in fact it will do them good to see that you have needs to.
Are there particular things that you could get the kids doing to help?
Certain things that your DH can do to help - obviously being supportive would be good but you can't control him...however you can control what you do. If there's stuff you need to not do, stop doing it. And if you want to go out in the evening for fitness or something, you don't need his permission....
hope some of that makes sense - good luck x