Sorry to hear about your loss. It is never easy. We had many cats and several dogs over the years. Luckily they all died of old age except one cat got hit by a car.
We learned over the years never to leave our animals at
the vet. We tell them we want to treat them at home. One time our dog was dehydrated and they taught my husband how to do it intravenously and they gave him the bags with the saline (?) water.
Animals often know when their time is up and they will hide somewhere. My friend's dog went into the woods to die.
Be thankful that you were the light of his life.