Sorry to hear your news but would agree about making tough decisions.
Last Thursday our cat was put to sleep after developing FIP. When we took him in 1st about a fortnight earlier he was ok but his stomach was distending and the vet said he would keep him comfortable as long as possible with steroids. His last dose was Tuesday and on Wednesday he was still eating but on Thursday we woke to find he had rapidly declined and was immobile with breathing difficulties. The vet was able to see him quite soon but I think we were trying too hard to fight the odds on Tuesday. We'd adopted him so we only know he was over 12.