I had to have our 16 year old Yorkie to sleep last summer (July 20 -I remember because it was easily the second worst day of my life). We had him for exactly 16 years and one day. I have had a lot of dogs but he was special to me so I kept him through the trying times of his deterioration. He couldn't hold it for more than a few hours so I usually came home to a mess and had to get up in the middle of the night to let him out. He couldn't jump any more so my husband built him doggie steps. But he seemed happy so we were willing to make adjustments. Then he developed congestive heart failure and suddenly his organs gave out. Within a few days and he was a totally different dog so we made a difficult decision. I took him in and rubbed his head and looked in his eyes while the vet administered the medicine. It was very peaceful for him.
I feel for anyone that has to go through that. It's the terrible price we pay for having them. I would focus on his quality of life. Is he happy? Is he in pain? That is where you will find your answer and the strength you need to get through it. Prayers for you and Gizmo.
And because I think he was a super cool dog and I can't help but brag about him even though he's gone. Here are some pics of Sammy Lee.