Toby99........ You are suffering from very real grief. We on this site understand that. Most of us have gone through what you are now experiencing. You are feeling guilty, and wondering why you did not pick up on the signs sooner. But the awful thing with kidney disease is that it is usually too late to do anything by the time the symptoms begin to show. You mentioned that Tilly had been weeing in inappropriate places for a while, and you are sorry that you did not get her to the vet about this. But, cats do that for all sorts of reasons other than kidney disease. Stress for example. And dont forget that just the day before the vet did not think that she had anything wrong with her kidneys. This is not a reflection on the vet,or yourself. It just shows what a sneaky, silent killer this disease is. It is a very, very common cause of death in cats of all ages. i would go as far as to say it probably the most common. I belong to quite a few cat forums, and it happens frequently.
Take comfort from the fact that although this was going on for quite some time, it is painless until the very end. But when the kidneys stop working, toxins begin to build up because they are not being flushed out by the kidneys. Tilly was not in pain, she just did not feel well. Take comfort also from the fact that although it was so shockingly quick for you, it was best for her.
Finally..... You must grieve for her but concentrate on the quality of her life, and not the length. It would not hurt you any less had she been twice her age. She had a wonderful life, full of love, and she could not have had a more caring family. Thank God that you had her and she had you for as long as it lasted.