A few years back, unfortunately I had to put another dog of mine down (also 12 years old, Cletus). He was in rough shape, and I was constantly coming up for excuses for not putting him down, or convincing myself that he was fine. Someone I worked with asked me, "Are you keeping the dog alive for him, or for you?" Made me think a little.
As tough a decision as it was, I probably let (or forced) him to hang on for a few weeks longer than I should have.
Again Tom, very tough decision, they truly become a member of the family.