We had a cat, Priscilla, for 18 years. When she turned 16 she started developing tumors in her lungs. We were able to remove them the first two times, but after that they were too small for the vet to remove, and we were in a tight financial situation, so we couldn't afford the $1500+ for the surgeries. She looked happy and healthy until about 3 months before the end, then she started deteriorating. She started having problems, first getting up, then walking.
She had lost all of her teeth except for one throughout her life, but she still enjoyed her food, though it had to be mashed for her to eat it. In her last couple months, though, she had a hard time eating, and feeding her would often take hours. She went from a healthy 9 pounds to just over 4 pounds in a few weeks.
We finally made the decision to put her to sleep when she started losing control of her bodily functions. We decided that keeping her around was selfish at that point, and that she would be better off not suffering.
The mile and a half drive to the vet that day was the longest ride of my life. I kept trying to think that I knew she would be better off, but I couldn't stop thinking that I was taking Priscilla to her death. I felt like an unwilling executioner that day.
The whole thing happened very fast when we got to the vet. We were assigned a room, and the vet came in after a few minutes. Doctor Kevorkian She gave Priscilla the shot, and within 2 seconds she was gone. That's one of the worst memories I have. As I'm writing this my eyes are watering and there's a lump in my throat fro thinking about that day.
Sorry about your dog H. You should find comfort in the fact that she's not suffering anymore.