A very sad day for you, but at least a difficult decision didn't have to be made and she slipped quietly away. I have a >15yr dog and she's struggling with multiple medical issues due to her age and at some point I'll have to make that decision. I think she's still got some quality of life right now, still has an appetite and can just about manage short walks. She loves snuffling in the grass and sitting in the sun so how do you decide right that's it - time to go? I hope she goes the same way as your dog, gently in her sleep.
It sounds like she definitely has some life still to live. It's when she isn't eating or wanting to enjoy life, that I think is the time.
She will most likely let you know when she is "tired".
For me,Tilly, my mini sausage, had cushings disease. She was beginning to deteriorate like yours but she was still wagging her tail and happy to potter around outside and enjoying her food.
Then on her last evening she suddenly had a huge blood clot come out of her nose.
She ate the blood clot but I rang my vet immediately and took her round and he explained that I could keep her going with medication but the blood clot meant that she could have a heart attack or stroke at any moment.
So that made my decision for me.
I knew her end was coming but I didn't want her to suffer any more and the thought of her having a heart attack or stroke would have been awful, for her and for me and for Maddie.
It was the hardest thing I have ever done but my vet was amazing though and he said that I let her go with dignity.
I was with her the whole time and she went to sleep knowing she was deeply loved. It was heartbreaking but having watched both of my parents die, if a death can be kind and gentle and full love, Tilly's was.