I am so sorry to hear about Rocky. I don't know if this will make you feel any better but knowing how painful and serious such a condition is, you were right to let him go. We had a cat named Motor who threw a clot when he was only 4 years old. He was Maine Coon, and heart disease strikes early in his breed. His back legs were paralyzed, and the vet recommended that we put him down. But my dear husband wouldn't hear of it, because Motor was so young. For a week and a half I provided round the clock care for him; exercising, massaging and warming his legs every 2 hours, force feeding him, putting him the litter box and cleaning him, and giving him Heparin injections and forcing pain pills down his throat. He started walking on his own, and the vet thought it was a miracle! With expensive medication and constant monitoring, he lived for 2 more years. I'm glad we did what we did, because he was so young at the time.
When we lose a cat I always console myself with the thought that it leaves room for another cat who needs a home. But since your husband is allergic, maybe you could get a dog when the time is right. Eventually the pain of your loss will be replaced with fond memories. I hope that happens for you soon.