I am so sorry for your loss. I am going through the same thing. I just lost my beloved Zippy to FIP last Friday. He was only four and declined so fast :-(
I have gone through a lot of painful things in my life, but this experience has been one of the worst. I miss him terribly. I live alone and am now completely alone :-/
I keep finding myself thinking I could have done something different and changed the outcome somehow, but I am working on accepting that's not being fair to myself. There truly is nothing anyone can do about this horrible disease.
I wish I could offer you advice other than to let yourself grieve. Heck, I'm a 43 year old man and I've been balling my eyes out in between bouts of serious anger that this happened to him :-(
I wish you the all the best. You are not alone.