Everything you are experiencing is NORMAL & It will pass as you go through the process. Just allow yourself to express your emotions and you will feel better with time & you will realize you did everything you could & now he is no longer suffering.
My 5-yr-old Pekingese was NEVER sick & even had a complete workup 5 months previously and he suddenly became sick. He was hospitalized several times over 2 weeks including surgery & after his death discovered he had a deadly form of cancer, hemangiosarcoma, that can result in immediate death.
I was devastated as he was always healthy. So despite what we do to keep our pet in good health sometimes we cannot always control everything and as part of the grieving process we have to accept this.