What Justice said really works. I used to have a dog ever since I was born and it died (cancer) right before I turned 11. He didn't live long but I was really close to him so I was obviously hurt a lot. I have a picture of him on my mirror and every time I look at it I'm not sad because I think about how much fun me and my family had with him.
You're sad now, but you're never going to forget the good times that you had with your cat or how your cat was with you throughout everything. But it's better that he didn't have to go through the pain and sickness anymore. And even though he isn't there with you physically, he'll always be with you, whether it be mentally or spiritually, in your heart.
I'm sorry you have to go through the loss, Garfield. May your cat rest in peace.
