Oh Sweetheart, i am so very sorry to hear this BIG HUGS.
Whenever any of us lose an animals we go through this what if, its natural, just think of it the other way round, if you hadn't of gone through with the op, and you carried on feeding him and giving him med's and he didn't pull through, you would be sitting there thinking, what if i had gone through with the op.......
You did everything and more than you could have done for him, everything you did was in his best interests. He knows how much you love and cared for him, and he has taken all of that to the bridge with him to start a healthy new life, but he will always be with you, watching over you.
I'm very sorry love. xxxxxx
Sleep tight Leonard x