Last December, me and my wife put down my wife's soul cat. Bella was 17 at the time she was put down. When Bella accepted you, she fully accepted you. She would come up to me while I was sleeping, and if she wanted attention, she would gently touch my lips with her paw. She did the same to my wife. She just loved unconditionally. My wife got her as a kitten, and she was there though my wife's divorce, new relationship with me, my wife's cancer journey. and everything.
When we were in the room to have her put down, my wife flat out asked the vet "Have we done everything we can to to keep her alive"? The vet reassured her we have done everything, and it was time to let her go. My wife held her in her arms as she was put to sleep. It's been almost a year, and a picture of Bella is my wife's screen saver.
It is just not easy, but you have the good memories and the good time with your dog. You just have to do the right thing. It's ok to cry endless rivers of tears.