It is incredibly hard and many adult children simply can't cope emotionally or really do not have the time. We are lucky inasmuch as our relative lives with us - I could not cope with the constant worry when he was living on his own - and we have practical help.
If he couldn't live with us he would have to be in a care home and maybe this is the best option for you, your family and your mum. Very hard decision to make (I know the time may come when we have no choice but to opt for a care home and we are dreading it).
I am very sad to read that your husband is not being as supportive as he could be, you need all the practical and emotional support you can get.
Take comfort in the fact that you are doing your best for your mum and make sure you get help, whether that be from your family or professional help. Make sure your mum is claiming all the benefits she is entitled to and speak to the council about any practical help available,