Man, I am so sorry to hear of what you have to do. Folks that are not "animal lovers" don't always understand but those who are can feel your hurt. Have had to do the same thing.
I view it like this.... we are blessed to have these dogs, cats, horses, goats, etc. in our lives and they become our friends. They are there for us and we need to be there for them. They depend on us to make hard decisions that are in their best interest and you are a good steward to do the "hard" thing when it is in their best interest. Glad you were blessed with a good friend for so many years......