You are describing the majority of mother-daughter relationships!
Quite honestly, my mother was always a bossy woman, but did it so charmingly that I was in my late twenties before I started to realise what she was actually doing, and took steps to prevent it.
In her old age, she turned into the woman ethelbags is describing, so I know precisely what it feels like. We put up with it as long as we could, until the day when my sister told her bluntly that if she wanted help, we were willing and trying to help, but her complaining all the time about the help she received did not make us feel like going on helping!
It took both my father and I over half an hour to console my sister afterwards and convince her that what she had said was nothing less than the truth and long overdue at that. My mother actually did try to be nicer to us afterwards.
So please, draw some lines as others have already suggested and don't feel guilty.
If your mother can afford to pay for more heat, then I suggest you turn it up, a little at a time. If she notices, tell her either that the weather is quite mild for the time of year, or tell her more honestly that you are worried she will quite literally catch her death of cold. Pneumonia is a real danger at her time of life.
If she is clinically depressed have a word with her GP. If she is just moody, try to ignore it, or tell her that she is making your life a misery as well as her own by her moodiness.
I know we were brought up to respect our elders, but respect does have to be earned, after all.