I'm so sorry
it is a very difficult time as a pet owner to make the right decision... i had to put a pet down a couple years ago and it was very difficult... the way i knew it was time was when he started giving up and no longer had a spark of happiness in his eyes, that was how i knew his illness was finally something he could no longer bear and have quality of life and my vet agreed that it was time.
Analog gave great advice, consulting a vet is a wise idea and may help you clarify your choice.
my heart goes out to you.
Sweetpea