Sorry you are having to deal with this.

I just lost my cat, he was young but had a chronic illness, and in the last few days he stopped eating, didn't clean himself ( his sister did it for him, bless), hardly moved except to sit by the heater and wouldn't even sit on our laps He also smelt really 'toxic' but that ws his kidney failure.

I think if you are asking the question you know the answer, esp if she isn't eating or drinking, apparently cats can't go for very long without eating. Our boy hated the vets so our vet kindly came to our house and put him to sleep, it was very gentle. they also sent flowers and a card to our house the next day, i couldn't be more impressed.

Good luck with a difficult decision, not matter what you do, you will feel guilty but its for the best if they are unhappy and in pain