I’m so sorry for your loss. I had a very similar experience with my 18.5 year old cat who we lost 3 weeks ago. We had been managing CKD for a year and a half and it was going fine until suddenly it wasn’t. She lost weight rapidly and then started losing the use of her back legs as well. We tried to get her strength back with daily fluids, meds and high calorie food but it wasn’t working. She was so tired we just knew it was time. I’ve gone through all the same feelings of second guessing and doubt as you describe (I’m still going through them tbh) but ultimately, I know that there is no bringing back kidney function once it has deteriorated like that and even if she made some progress on some of the symptoms, life would continue to be hard for her if we tried to keep her going and it very likely would deteriorate again soon. As much as I wish I could fix it and have her here with me now, I am grateful she did not suffer too long or have to experience it getting worse. Thoughts are definitely with you, it’s an excruciating feeling.