I feel like I put my dog to sleep when maybe I could have done more?

You shouldn't feel guilty. She is in a better place. You ended her suffering. And don't say you won't get another dog. Your dog(in doggy heaven) is probably thinking "Don't say you don't want another dog! I want another dog to play with my favorite toy, to sleep in my bed, to be fed like you fed me. I want other dogs to be as privileged as me, to have a home with someone as loving as you. Don't let me be the only one in your life!pant pant pant"