It's an awful feeling. My condolences.
Sounds like your cat had kidney failure. If the cat is old there's nothing that can be done. One of my cats just recently pulled through kidney failure. He was having trouble urinating, it was bloody when he did so I knew he had an infection, so the night before I took him to the vet I decided to give him a tiny bit of ibuprophen to keep him comfortable. He was having some pain so I had to do something.
Yeah,
bad bad thing to do! That wound up putting him in advanced kidney failure, so bad that in the morning I had to put him in the tub because of the intense amount of blood, like a murder scene. I washed it off and more kept coming. The tub looked like I slaughtered an animal in it. It was surreal. When I got him to the vet, hours earlier than his appointment because of the emergency, they said he probably wouldn't make it through the night. I didn't even realize it might have been what I gave him until I looked it up online, just to see. The realization of what I did after reading about it online hit me like a sledgehammer in my chest.
He wound up pulling through, $2,000 later, but he's still young. He's only 8 years old. That was a month ago and he's good as new now, and more cuddly than ever (same with his sister who had bone cancer a few years ago and needed an amputation; she's happier and more cuddly than when she was a kitten).
NEVER give your pets aspirin, acetaminophen or ibuprophen! Even if it's a tiny amount and the intentions are good, you may wind up killing them. Cats are about twice as susceptible as dogs. I thought I was doing a good thing, helping him with his pain until he could get to the vet the next day.
Umm sorry for telling that little story in your thread about your cat, not mine.
The message I give is vitally imporant though! A little wedge of one ibuprophen pill wound up costing me $2,000 and very nearly killed my cat, so don't let that happen to you!It's gonna take time for you to get over it. I recommend after a short time has passed, perhaps late spring or summer, you go to the local animal shelter and see if any of the cats there pick you. Yep, they pick you, not you them
You'll know it when they do. There's room in your heart for a new cat even if you don't know it right now, and afterward you can comfortably realize that your cat indeed died of simple old age which cannot be prevented. 20 years is a long time, but there's another cat (or maybe more than one) that will want to spend it's life with you, out there somewhere