I'm sorry to hear about Shelby but he knew the human touch. You loved him and he knew it and whether he made it back to you alive or not, he still carried you w/him in his travels wherever he went. Sometimes we can only save them for a day. Many a cat came to my porch to die. I'd find them the next morning, sometimes frozen, sometimes just plopped down in a shelter that they barely were able to make it to. You did your best and that's all we can do. Don't punish yourself w/regrets. They serve no purpose other than to help us to make better decisions in the future. A candle burns for you both.