Robert, I am so sorry for you and your families tragic loss - it is never easy to say "good-bye" to a most loved and cherished companion. For me, this is truly "my hell on earth" when a decision such as this must be made. It really hurts you to the very core of your soul to do this, but we cannot let our friends suffer - we must be strong for them also.
You come home from work and they are at the door waiting for you "wagging their bums off" and you see so much love in their eyes. My black lab, Apollo, had cancer and we also had to euthanize him - he was almost 13 years old. My friend was looking at me while I was signing the paperwork and I just hated myself. We had the rest of the day with Apollo telling him how much we loved him and I had the Vet come over to the house later in the afternoon - Apollo did not enjoy going to the vets and I wanted him to be in his own home when he closed his eyes for the last time. I actually cried everyday for the next month and every now and then, I shed a tear for him to this very day - Apollo has been gone for 10 years now.
Cherish and hold dear the moments you shared together for they truly are a gift.