Oh dear how I feel your pain, for you and kitty. Nothing is more painful then having to say goodbye. I have cried an ocean of tears for several of the kitty's I raised and had to put to sleep. I know no other pain of heart then when you have to put kitty asleep. I have had to make choices I wish that I didn't have to make. Pain is so awlful. Think of it like this, think of your worst pain that you have endured and assess that with what kitty is having to endure. It may make the descion a little easier to make..
I know you have had him for 18 year, happy years, I know your grief and reluctance of putting sweet kitty to sleep but like dear Scully posted was so true. Her suggestion was beautiful. I am crying for you and I feel your anguish and I know it is so hard. My babies use to do all the crazy cuddly things your kitty does. I know that they are better off in Kitty Heaven. You can ask your vet to have kitties remains after he is put down (crying) make him a little coffin and have a family ceremony and celebrate kitty's life. Pictures and happy times. Then you can put him in a place that is special to you and to him. He will always be with you in your thoughts and memories. Give yourself time to grieve. With time the pain will go away, but you will always keep him locked in your heart. I have so many wonderful memories of my kitty's that if I dwell too much about it I begin to feel sad and cry and that is good because you know that you have feelings.
I would make a special scrapbook of kitty and make it so special that you can keep on the coffee table for all to see the wonderful things that kitty brought to your life. Then one day you will move on and realize that kitty would want you to do that. Somewhere along the line you will find that kitty has a permanent place in the family tree.
He'd want you to find happiness with another furry friend, he won't mind. Never forget him and cherish all the memories and pass along his legacy to all who want to hear.
I love you and hope that you can come to peace with this. God bless
all my love
Orangefatcat xoxox