My 19 year old kitty was in kidney failure. The vet suggested we give him sub q fluids....(fluids under the skin) it made him feel better, seemed to keep him a little more active and it kept him eating....he was with us longer than we expected. But he also let us know when it was his time. (((Hugs))) to you....losing a pet, especially when you have had one a long time is so so sad....there is no other pain like it. I have lost 9 pets in my lifetime so far. Hopefully your cat will bounce back and regain some strength and you will have her for a little longer....21 is pretty awesome.