Decisions on euthanasia are very personal ones. Some will go to the last breath and others feel it is kinder to let the pet go to spare them discomfort.
I think at this point you have to ask yourself if your pet is happy and comfortable or able to do any of the things she likes to do.
When I have to make this difficult decision I try to measure the animal's quality of life and disregard my own heartbreak over the loss. I try to base the choice on the pet's happiness and comfort. You know your pet best so can best judge if she is still able to do what she loves or not or is uncomfortable.
If you do have to part with her I recommend the site http://www.petloss.com/ and the rainbow bridge story which may bring some comfort.
Hope this helps you!
Cat Health, Behavior, Care Expert
30+ years cat owner, rescue, breeding, study of behavior & health care