Hello,
I am so sorry to hear about your kitty. Eighteen years is a good long life for a cat and you have done a great job giving her that time. In order to be able to diagnose kidney failure with bloodwork, they have to have lost at least 75% of their kidney function, meaning they are left to operate on only 1/2 of a functioning kidney mass. If she were a person, she would need to be on dialysis or she would surely die.
The kidneys are a big filter for the body's toxins. When the kidneys stop working, those toxins build up in the bloodstream and cause nausea and even gastric ulcerations. They feel very weak and are very uncomfortable.
This is not the sort of illness you want her to slowly die from. They get progressively sicker and more uncomfortable. The kindest thing to do is to have her humanely euthanized. I think you might feel worse if you wait and begin to see her suffer more. I know it's a hard thing to do, but you sometimes have to let them go because it's the kindest thing you can do for them even though it is hard for you.
I'm very sorry... My thoughts are with you.
Cat Veterinarian
10+ years practice experience as a cat/dog veterinarian- special interest in behavior