One of our dogs also got cancer and was put down. He was the healthy one, which was really ironic.
The other one had serious problems, particularly arthritis, and the vet gave him 6 months to live. We put him down 6 years later, 2 or 3 years after the one with cancer. It didn't seem like his time to me either but it wasn't really my decision and I knew it so I just let it happen.
At some point last year the amount of my life I has spent with that dog passed the 50% mark. It's weird because there's all kinds of milestones I can measure my life to, and yet I feel like a different person for transitioning from, "most of my life with that dog," to, "most of my life without that dog."
wait how the forget did I make this about me

I was just trying to share my experience of my own dogs being put down, one from cancer. I guess the point is that everything dies and life goes on.
But I'll tell you right now, some day I
am going to have another dog. Insert your own message about the joy being worth the pain of loss or whatever.