How do you say goodbye?

about 18 months ago my dog was given a tragic diagnosis. He has chronic renal failure, there is no cure, just a long decline ahead. He's had ups and downs over the past year and a half, but he's having more downs now than before, and it seems like we might be nearing the end.

How do you say goodbye to a dog who you think of as a child? A dog that was your first joint venture with your future husband? A dog that taught you how loyal and loving you can be for another being that takes you just as you are, no matter what kind of day he has? How do you say goodbye to the pup that was there for your wedding planning madness, and there for the photos after the ceremony? The dog that curled up on the bed with you when you brought your first human child home from the hospital? How can I imagine life without his happy dance when I come home, and his amazing sense of "cheese" that brings him running if I open a new block?
How can I expect to sleep soundly when his reassuring snort and snuffles no longer fill my bedroom with a constant rhythm? When his relaxed body is no longer sprawled across his bed right in front of the room heater luxuriating in the radiating heat?
The prospect of a Mao-less home is sickening. I can't imagine not having to dodge his sturdy body when I want to pass through a doorway he just happens to be inspecting.
This morning we went for a walk together, just to have some time together, he took in the scents of the neighborhood and I was almost able to pretend that he was well, and that this upcoming loss was nothing more than a horrible nightmare that disappeared when the sun came up.
How can I go on when he can't? I don't know. I just don't know.