You have taken a giant step toward healing by accepting the fact that no one can be 100% sure what happened to your little boy; though they are pretty sure they know.
I'd say that's real progress for now. Time is merciful.
You are not a failure. You are not a bad Mom. This was something beyond your control. You did all that you could by rushing him to the Vet. Exactly what I would have done in the same situation.
Be as kind to yourself as you would to someone else facing the situation you were in. Would you blame them? Think them a bad person? I don't think so. Grant yourself the same kindness you would a total stranger.
Peace be with you and well done on accepting the answers the Vet has given you with the information he has. It sounds like he has made a real effort to help you search for understanding.
