Citrine, I wish I had seen this post sooner. I had a kitty die at a year old of FIP. His name was Bo Jackson after the baseball player, and he was the cutest fluffy tailed gray/brown striped tabby.
He had all the symptoms, so I took him to the Vet having no idea what it was. The Vet tested him and just put him to sleep. No phone call or anything. Then I called them at the time they said for me to, and they told me they had put him down.
I started asking them what FIP was and--and this I have never done before or since in my life--in midsentence broke into heaving sobs! I couldn't stop, which was embarrassing with the Dr.'s assistant holding onto the phone talking to me. My heart was broken, and I sobbed on and off for 3 days. I couldn't help myself. I felt so sorry for this little thing that didn't even get to live his life out. Life is not fair!
So, I have been thru the FIP thing. I don't want to go thru that ever again, although it won't stop me from getting another kitty as I enjoy them so.
My sincerest sympathy for you on your loss as I know it is hard to go thru. Time heals the heart..but you won't forget this, I'm sure.