I am so sorry to read of your loss of Duncan. I, too, had to put my best friend, Jack, down due to a twisted intestine. He was a 17 hand Appaloosa gelding that was 17 yrs. old. Had the vet out and he was on the phone half the night with the veterinary school on things to do for him. I walked him all night long---he'd drop and I'd pull and scold him to get up. I'd give anything to have him back.
It is times like these that make us as the caregiver to put our feelings aside and do what is best for the animal. That is all they ask of us in life--is to help them. And when they are beyond human help, then we must help them so they do not suffer. Duncan did not deserve to suffer---he is free now, to run at full speed or to sleep under a shade tree near the Rainbow Bridge. I truly believe animals have souls too--much like human souls only an animal soul. Duncan will be waiting for you someday and will welcome you with ears tucked forward and shiny bright eye on your arrival!
Be kind to yourself. You are an amazing caregiver to be able to put Duncan above your own human feelings for him. That is awesome for Duncan to have someone to have cared for him as you did. Just know our thoughts are with you in your loss of such a good friend.