I'm so sorry for your loss...My family and I too recently...
had to put down our beloved 17 year old kitty Billy, in June, also from kidney failure. He had Chronic Renal Failure, which he had been struggling with for years.
Reading your post, I know exactly how you feel. We have 3 other kitties, and I have had cats in my life since I was 7, (am 47 now), and I loved them all very, very much, but Billy was so very special to me. This cat and I had a real connection...always did, right from the beginning. I swear we could read each others minds. Billy was a treasured member of our family, and we all loved him so much. It's been almost 6 months since he died, and I still miss him every day. I don't think I could I could miss him more if he were human, and I will miss him for the rest of my life.
My prayers are with you...Hang in there!