I feel really sorry for you and your cat. But I would let nature takes its course (of course, when I say that, I do not mean that you shouldn't see what a veterinarian can do for her). Don't try to hurry along her death. She only has one life to live (not 9 like we joke about cats), so every moment is precious.
From what I know and understand about Cats is that when they are ready to pass over, they will let you know in their own individual way and in your heart you will know too!
It seems to me that most animals, when they are ready to die, will stop eating.
You need to teach your cat the bl<x>inking eye method..2 bl<x>inks means "yes please--kill me"
I've been through it a few times with cats. As long as they are functioning (using the litter box) and reasonably comfortable, I think it's ok. When they get beyond that, I took them myself (and held them) while the vet gave them the injection. Not an easy thing to do but that's the cycle of life.
My cat did-held on for a long time-they will start to howl in pain-AWFUL and sad. When they reach that level. hugs
YW : ) It was-I immediately brought him in and they put him down : ( I stayed while they euthanized him. He jumped up and hugged me-HEART WRENCHING DAMMIT! Good luck.