I put down my dog and it felt like the right decision, but in the long run was it?

When the animal's quality of life and comfort level are gone, there is no reason to let him suffer any longer. Animals don't understand why they're in pain, and they will fight illness as long as they can but they know when it's time to let go, and it was time for your dog. The true test of character here is one that you ped with flying colors, because it's MUCH harder to say goodbye and end your pet's suffering than it is to be selfish and keep him alive when he's in pain. I'm not a religious person, but I would have to say that if God was going to punish you for anything, it would be for letting another living thing suffer...that would be like punishment for the animal. Whenever someone makes that heartbreaking choice to put an animal down, they are truly an unselfish and strong individual. I think this dog was meant to be with you so that his life could end with someone who loved him, and not in a lonely cage.