Loss of our 9 year old cat

We adopted Puffin from a foster carer and he was such a beautiful snuggly boy. I have chronic pain and he knew when I wasn’t well and always looked after me.
He had some anxiety so we helped each other. Apart from teeth issues a couple of times, he had no other health problems and we thought he would live to an old age. At the end of July we had a tragic diagnosis of myeloma and the vet said it was very unlucky as it’s rare. We treated him with prednisolone a steroid and he started eating again and seemed ok, but deteriorated at the beginning of November. We knew he wouldn’t survive, but didn’t realise the emptiness we would feel. He was a special cat, you could say an emotional therapy animal so didn’t deserve such an awful diagnosis. Now he’s not here, we feel it every morning to late at night. And, I feel very angry right now too. We have organised to have a decorative urn and a paw print to remember him, and may also plant a tree. Last time we adopted fairly soon afterwards as our house is very quite without the footprints and meows. The sense of loneliness is very high, and just like none of the daily stuff matters. I did read it might be good to write a letter to our cat that passed, so I might try this.