I went through this same thing with my darling Tink on April 27th of this year. She was a Jack Russell, a month away from turning 13. She was playing and happy and being herself and then suddenly she wasn't. She wouldn't eat, and we knew she was in pain. I took her thinking it was just an infection or something. The vet ran the test and told me she was a very sick little girl. I was also given the option of hospital care, but with not much hope of it helping. I made the heart wrenching decision to help her go home. It was the most miserable day I can remember having in a long, long time. Even now I will cry because I feel guilty and think what if I had gotten her the treatment and she could still be here with us. But in my heart of hearts I know I did what was best for her. Just as you did.
A side note-my 2 darling dogs I've had as an adult was my Tink and a beagle, Phoebe(lost her when she was also 13). I mourned a lot after Tink. My DS,20 happened upon an add for a puppy -a Jack Russel/Beagle mix. He picked him up and brought him home to me on what would have been Tink's bday. I wasn't really ready for that, but my DS said "Mom, I think they sent him to you" So of course I kept him and love him, how could I not. His name is Willie and he's such a little charmer, he really has helped me.
Hugs to you-I know how heart breaking it is.