I am so sorry. I had to have a cat put down for the same conditions, this one a Russian Blue. He was gorgeous, gentle and so easy to love. My companion. I still miss him. Had a maine coon, which died last summer. God in his infinite wisdom gave us our pets to be a part of our families, and it makes it so hard when they leave. You will be in my prayers tonight.
We have two orange tabbies and one British Blue. The British Blue is the one that is allergic to poultry. She used to get sick every time she ate. We took her in because her previous owners got tired of dealing with it. We used to have to take her in for steroid shots every 6 weeks and even then it only helped a little.
Now that we know what the problem is, it is a lot better. Not perfect, but a LOT better.
You are so right though that it is so hard when they pass.