Oh how my heart is breaking and the tears falling as I read your posts. I'm so sorry this happened to your family and your pup.
Dogs are so innocent. Your little guy doesn't feel like you let him down. He is so innocent that he don't even know what that is. All he knows is that you cared so much about him. You took him to the vet because you care about him. He knows you are forward thinking and questioned taking him there. But, you always question because you are a caring mom. That's what caring mom's do. He knows it isn't your fault at all and he would never blame you.
What happened to you isn't fair.
I wish we could know when we are going to pass and how we are going to pass. If we did, we could love more and give more. Losing such a sweet innocent pup at old 6 years old is devastating. Not knowing what happened, heaps so much on top of it.
Your pup is happy now, though, you should know this. He has no pain and will never have pain or illness ever again. You'll get to see your pup someday. When that day comes, the time from now until then will feel like only a second because when you are reunited, you will not remember your grief and the loss you feel will be gone.
I know it is cliché to say that time will heal you. I don't know if that is true. I cry every day, sometimes for hours on end, missing my sweet girl. First my tears were disbelief. Then they were guilt and regrets because of the vets and our decisions. Now, I somehow feel that crying keeps her alive and if I stop she will be lost from me forever. Somehow accepting her death means she will be gone from me and I don't love her. I'm trying to find a way through this to honor her and respect her but it is so hard. I have been spending time with my other dogs and that has been helping. They can never replace my girl, never. But they are giving me love in a way they never did before which I really need right now. It eases my pain. Maybe your pups can help you too.
Just for a moment, feel your pain and sorrow. The is how much you love your pup. It's more love than you ever knew you could feel. Your pup loves you back just as much. He always will no matter what.