This was a meaningful thread for me and I got a lot of value out of everyone's stories, as well as the PMs. Since it is done I'll share mine.
The day I wrote the OP I made an appointment to put Abby down the next day. That evening we took the kids to swimming lessons, and when we returned home I opened the door to a dog with wagging tail and a ball in its mouth. My heart was in a submission hold and I tapped. I called the vet and said I didn't know what I wanted to do but wanted them to take a look at her so I could discuss whether or not this needed to be done now. One thing that had made this decision so challenging was the up-and-down nature of her cancer in the last stage of her life. One day she would seem bummed and in pain, the next she'd want to go swimming and play fetch. For the sake of the dog, I felt it was better to be a bit too early than too late.
So we upped her meds and gave her another week. That week she just seemed more doped up, and not in any less pain. In fact for four or five days she just kind of seemed depressed, and didn't get up and do much at all outside of eat and use the bathroom. On Friday we decided it was time, and made an appointment for the following Monday(yesterday). Over the weekend she perked up, we played fetch and went swimming, and we loved on her and fed her bacon and sausage. Knowing that the end was coming was heart wrenching, but I felt we were doing the right thing. Her bad days were outnumbering the good ones.
Yesterday we did it. As I blubbered over her she nuzzled into my wife and I, and I was overcome with shame that in her last moment when I was to be comforting her, she was comforting us. I buried her in my field overlooking the Elkhorns. The last thing we did together was take her swimming, where Clancy Creek pours into Prickly Pear Creek. Not having any weight on her joints while swimming in the water she was burning bright, and I threw sticks for her for an hour. She'd lose track of the stick and swim against the current, with little more than her tail for propulsion, she was stationary in heaven and hopefully still is.