Sorry for your loss. I've been through that. My last dog was put to sleep at age 12 and a half. Anal sack tumors (?) or something like that. Made the decision when the vet said it was time. Waiting would have been agonizing pain and a slow nasty death. Didn't want her to suffer any more. Was with her cradling her head in my arms stroking her head while the vet gave her the shot. She just drifted off calmly to sleep and that was it. That was back in 2006 and we haven't got another dog yet. The wife gets too attached to them and then they leave to soon. It really was the loss of a family member.
