I feel awful for you, but I think you did the right thing. You knew that your kitty was in severe kidney failure. You chose not to put her through the stress of taking her in for IV's, medication, all those stressful visits.
I feel kind of guilty sometimes, because my cat was on the brink of death a little over a month ago. We thought he had cancer, all the tests that we had done pointed to it more and more. But at the moment when we thought we were on our last days, they found a blockage that was fixed with surgery, and he is OK. As happy as I am to have this happen for my cat, and trust me, I am so thankful, I also felt....guilty, a bit, or some other feeling, maybe not guilt, but I don't know what, that I was reading all of the stories from people that were losing their cats. Here I was with a miracle, and so many didn't have that happen. I truly felt bad for all of those people, and for you. Still, I think what you did, making that decision to not drag her life on, was a good decision. I had already decided that if my cat did have kidney failure, or liver failure, or he did have cancer, I was going to let him go, and it was going to be fast, because I could not bear the thought of seeing him go through all that he would have to go through. As hard as it would have been for me, and as miserable as I would have been, I still have no doubt that I would have made that decision, to ease his pain. My mom had a cat with kidney failure, she had to give her sub-q fluids every other day for a long time, years maybe, and the cat was a skeleton in the end, and kept alive for far too long. I promised myself right then that I would NEVER do that to my cat. As sick as my cat was, his blood work showed no signs of kidney or liver or pancreas problems, all he had was a high white blood count, so it was really hard for us to make a decision to let him go without really knowing what was wrong with him. If they would have told me it was something really bad, I was prepared to let him go and not drag his life out just to keep my from being sad.
Please don't beat yourself up about your decision. I think it was a very selfless thing that you did, not putting your cat through that ordeal. I understand your feelings too, reading about people who are doing treatments and their cats are hanging on, or they find out the problem, like in my cat's case, and they get better, and yours is gone. That must be really hard, I totally get that. I'm just really sorry that your girl is gone. She was very pretty, and it is clear that you loved her. Don't second guess yourself. She is not in pain and she did not have to have her last months with you being poked, prodded and medicated. I'm so sorry for what you are going through.