Aww, I know exactly how you are feeling. I've had to put my 11 years old black cat, Chad, to sleep after the vet discovered cancer in his mouth (I thought it was a rotten tooth, it just came on quite quickly) and six months on I still miss the little dude. Makes you wonder how something measuring 18 inches in size could take up so much room in your life.
However, next door's 1 year old black cat, Bandit, has taken upon himself to be the next Chad as he spends more time here at my house than next door . . . but the spooky thing is that his mannerisms and habits are exactly the same as Chad's so at least his spirit is still very much alive.