When do you know that it is cruel to keep your dog alive because you can't let go?

Hi

I have a 17 year old collie/ husky mix. I have owned my dog since he was 6 months old. Snoopy is very much a family member. He didn't much like when we added our daughter to the family but got over it as soon as he could follow her around and steel cookies from her hands and birthday cake from her at the dinner table.

He is having a lot of issues now he was fully trained to go outside and now is leaving messes all over the house. We walk him now almost hourly since my husband works from home and it still doesn't help.

He use to bark when he needed our attention if we were busy and now rarely barks at all, and when he does it is a funny bark almost like a dog who has never backed at all getting there voice.

We noticed that his top front teeth are missing and haven't grown back in. We have switched food to a softer choice to make it easier for him to eat. Sometimes he even cry's when he has to go to the bathroom outside like it is a painful process.

He walks a lot at night as it is difficult for him to sit or lie down, he bangs into things constantly. He looks like a drunken sailor when walking as he doesn't walk straight.

He has had growths on his outer eye that the vet won't remove so they break and we constantly clean them as they seep fluid.

Sitting is painful for him and trying to get down the stairs rarely works for snoopy he falls flat and we have to pick him up which he hates.

He growls a lot more at all of us, we believe its because he doesn't feel good.

Two nights ago he got stuck over the toilet seat and won't let us help him up so we watched and assisted a little at a time until he was free.

We can't let him outside unattended on our 3 acre property as he forgets where he is and can't find his way back to the house. We now keep him on a leash and walk him.

I don't want to put down snoopy, but for the last year this has been a conversation that we all have had.

I know he won't live forever, but when do you know that it is cruel to keep him alive because you cant let go.

Sincerely,
Brigitte