I don't venture into these parts often but I saw the title of the thread and I had to click on it I lost my dog like 2.5 years ago is it already? Damn, still doesn't seem long ago tbh. Very similar circumstances as your cat.
The best advice I've ever gotten about this type of thing is that it feels the worst right now but only time will help heal. You'll always be sad about her last days but time will make things easier. Celebrate the good times, my dog was 15 when he died, we had him since I was just a kid and my brother and I took him with us after hs. He was a great dog and the best thing I can say is that think of the good times. You had 20 years with this one, must be some pretty cool/funny stories. Laugh, cry, celebrate her life and try not to think of the end. There's an end for everyone and everything, life is finite. Remember the good times, know that you gave her a good life and she loved you for it even if she couldn't'tell you she did.
I now have a cat who just turned a year old and even tho it's the first cat I've ever had [my last dog was a male so I got a female cat to get as far as possible from my ole buddy as I could so I didn't feel like I was replacing him]. Cats are fucking weird man, growing up with dogs it took some time getting used to her personality and her scratching my shit up being all catty and shit. But she's great wouldn't give her up for anything She's smart and I know one day it'll be similar to my dog's ending, but that's a longtime from now, hopefully, and it's just the cycle of life. I feel for you and remember think of the good times, try to forget about the end. There are a lot of times before that for you to look back and smile upon.
Anyway hope this helped a lil bit. Sorry bud, RIP to your cat.