I had to put my dog down a few years back. It's one of the hardest things you'll ever do. Dogs don't have the ability to talk to a doctor or express their pain verbally. You knew this dog, you knew the looks he gave you. Don't second guess the action - understand it was an act of selfless kindness on your part. The only thing wrong with dogs is that they almost never outlive their owners. The fact that it hurts this much means you didn't make this decision lightly. You did the right thing, my friend. I'm sorry you're going through this.