
Been down that road a couple times.
The first was a spanielly mutt that was dumped on our tree lawn while my (now) wife and I were first living together. We figured we'd keep her over night while we found out what to do with her. We had her for 18 years... The last 5 or 6, she was running on Rymidal(sp?) for severe arthritis in her hips. Finally, the vet told us it was time to let her go. Reluctantly, we did.
We got a pair of golden mutts from a rescue place to replace her. One of them developed Addisons AND megaesophagus concurrently. It was shortly before I was "retired" out of Cisco in 09. Around Christmas, the vet gave her about 3 months to live. With rather expensive medications, she had reasonably good quality of life until June of 2012. Her sister had died the month earlier of bladder cancer, which popped up and killed her in a period of 8 weeks. Again, it became evident that it was time to let go again.
It is very hard, but we owe them that last responsibility.
I'm sorry for what you have to go through.
"Religion, n. A daughter of Hope and Fear, explaining to Ignorance the nature of the Unknowable."
~ AMBROSE BIERCE
(1842-1914)