My own decision-making path began when I was eighteen, and had left things too long for my adored elderly cat who I had to carry to my vet, and see him put down on a clinical consulting table. It was such a shock, and so sudden. The grief and guilt then were immense, and I vowed to change things if I ever achieved my dream of becoming a vet.
Now, after 30 years, I have a better ability to judge a pet's Quality of Life, and I can offer advice, but the tough and kind decision has to be yours to take. Above all, listen to your pet.
As a guide you could note the three activities that your pet most likes to do:
If one of these normal activities disappears more days than most, it is a concern. If two activities are hardly there, you are being pushed to make the decision. If all three disappear, your loved pet is making the decision for you. In addition, there are more detailed check lists where you can monitor activities and signs to give you a more objective direction.
Clearly, the stopping of eating and, in particular, drinking, are red flags, please act on those quickly. If you haven't done so, a trip to your vet for a check-up and advice would be sensible to rule out a condition that is curable or treatable, like bad teeth for example.
If you aren't sure it is the right time for your pet, please consider a read of the relevant link(s) with useful and impartial advice for you below.
You are very welcome to give me a call. As I have mentioned before., I can't comment on your current veterinary treatment for ethical and other professional reasons, but we can have a chat about what is happening, I am always happy to talk.