Hey puss, sadly a lot of us on here have been in that situation - I know I personally have many times.
It never gets easier when the time comes to make the decision to end the creature's life (perhaps it was always gonna be harder for me 'cos we used to take on rescued and mistreated animals) so unfortunately I was a regular visitor on a mercy mission to the vets .
When the quality of life has gone - then's the time to do the right thing for the puss.
Sorry, that hasn't answered your question, but I want you to know that there are folk thinking about you and your very dear puss.