I'm sure you know that many of us have faced that grim decision and the grief of losing a beloved companion. Try not to second guess yourself--you know that you did your very best to make the decision that was best for Butch. Believe that your abiding love for him lead you to the right decision for him and at the right time.
My first dog, Claire, was with me for 17 years. I treasure each of those years in her good long life...but I think the fact that she had shared such a long and important period of my own life added another dimension to my grief at loosing her. Butch was with you for a similarly wonderful long time and I think that makes the loss particularly poignant..
I prayed that I'd know what to do for Claire and I've always been grateful that the time to help her to the Bridge was very clear. Her little body just couldn't go on. I think that the failure of Butch's body told you what needed to be done, and you did it in love. Grieve when you need to. Post when you need to.
Lucy