So sad for you.
Our vet told me I would "just know" when the time had come to to take 17 year old Tom to the vets for the goodbye injection.
He had increasing problems with kidneys and liver and was eating less and less, weeing enormous, smelly quantities and his poo was horrendous.
One day I came downstairs and he had been sick/pooed and there was blood in both.
He tried to follow me upstairs but was too weak and started to fall back down.
It was utterly heartbreaking but I knew then what the vet had meant. I "just knew".
DP couldn't do it and the DCs said goodbye at home and I took him to the vet. It was very peaceful and dignified but I was in bits. The vet was incredibly kind.
That was three years ago and we still have his photo and his collar on the mantlepiece and DD made a special scrapbook.