My buddy is gone :(

Our 5 year old leo passed yesterday. BLIZZ suffered from MBD. I tried to help him but he had it most of his life. My grandson was the first owner and I took him in last year. He was my buddy. In the past week he just stopped eating and lost so much weight, stargazing, and opening his mouth wide, labored breathing. I got an emergency vet visit. They said he was severe. xray showed his bones very thin and fluid in his lung. They advised he be euthanized as he may be on pain. I was alone and a complete mess. I did not want him to suffer. When they returned him so I could say goodbye he got very active and I said I may change my mind and could they treat the pneumonia and they said it would be too much for his body to handle and that he was probably in pain. That he was looking more alert due to giving him oxygen. I kissed him and said goodbye. I can't stop second guessing my decision. I'm sick with guilt.