Time heals wounds, and I hope with time your pain is eased. I know the feeling, as we had to take our cat to the vet in June for the final time; she had cancer and couldn't keep anything down any more. She had outlived her prognosis by about 3 years already, though. What a cantankerous old queen she was, ruling the roost and bossing her humans.
But a month later we have been given another cat and she is the sweetest, gentlest thing you ever met; she seems to think I'm the love of her life and is constantly on my lap, like a dog. Didn't think we would do this again so quickly, but it was the best thing we could have done. We're feeling mighty happy again with our new fur kid now. Hope you can make a new friend soon, too.