Tryingmybest, I wholly agree with what others have written.
Yes, your choice to give up work - but her choice to agree to move in with you - you each 'face up to' the consequences (forseen or not) of those decisions.
Imagine you were 'simply' out of work, you may well take in a lodger to help with finances, it's been done for centuries.
Well, in my mind, your mother is your lodger.
Her partners previously looked after her finances without her being bothered, you now have that responsibility, whether she completely understands that or not.
Have a look round at what is a fair rate for a room in your area to get some idea of what you can reasonably 'accept' from her.
Check out info about taking in lodgers eg
http://www.lodgers.com/taking-in-a-lodger.html
Consider equally sharing bills for water, gas, electric, phone line rental, food, cleaning products etc - or at the very least add on a percentage for the additional amount she will use - say 25%, then maybe in time compare the last year with the year from when she moved in.
If you as an adult had moved in with her you would have paid your way.
And remember, she pays for anything directly to do with her eg day care, extra waterproof bedding, taxis to appointments - that's what the AA is for.
What Pete R wrote is shouting out at me:
At the time I would have thought it abhorrent to do the same with my Mom but looking back I was stupid not to.