I'm so sorry for your loss, I recently lost my dog. I'd had her seven years, through breakups, eating disorders and everything in between I had my dog. She got a tumor and was just to far gone, I took her to the vet and he promised to do everything he could but she died a few hours later.
It's been about 2 months but the pain is still fresh and very real. I wish I could give you a hug right now or say something to help you, but the best I can do is promise you that I know what you're going through and you're not alone.
I'm here if you ever want to talk. I know the guilt that comes with losing a pet, I hate myself for taking my dog to the vet and her dying there without me but I can't change it. I'm sure you're angel of a cat knows how much you loved her and she'll always love you, animals may die but love never does <3