I am sorry for your loss, and if it's any small comfort or consolation, I know exactly what you went through. I lost my Toby Ann in March. She was 16 1/2. It was kidney failure. Fwiw, I understand this is how most older cats meet their end.
Toby quickly began losing weight. The doc suspected CRF (Chronic Renal Failure). We tried some meds, but they were of little use. Finally she became so weak and dehydrated from not eating and drinking that I made the appointment. I did most of my crying before hand, cuddling her at home and knowing this was it.
The veterinary staff was so kind and compassionate.
I got a copy of Rainbow Bridge from someone, and made the mistake of reading it at work. I was a mess. It's still hard for me to deal with... she was my baby and my Princess. But I do believe one day we'll all be together again. But for now, they're playing and doing kooky cat things like they used to, somewhere else.