Its so difficult, but at least you were with him at the last.
My 16 year old cat, Lacey, ran under my OH car last July (the morning after i had put a deposit on Taz - i couldn't help thinking it was my fault) we thought she had broken her back leg, but when they got her on the operating table to amputate they xrayed her and found two breaks in her pelvis and they phoned me to see what i wanted to do as if they changed the operation to mend her pelvis she would have to be caged for 8 weeks, followed by another op to amputate her leg. It was the hardest decision i had ever made, she hated being shut in a room, let alone a cage for 8 weeks - i couldn't do that to her at her age. They said if i wanted they would bring her round, and i could take her home for the evening and then take her back the next day to have her pts - but that just seemed to hard on both of us - in fact i am welling up now just thinking about it. its taken me ages to come to terms with it.
take time to get over it, and think about the good time and how lucky he was to have been yours