I am really sorry Destroyer.
I had a very similar experience to you to the end of last year.
We had a cat called Harry. We saved him from being put down, bought him from the SPCA in late 2000. He escaped the box and got loose in our car on the way home, hence his name-- a reference to Harry Houdini.
I'm 16 at the time of posting, so he had been a childhood pet that I've known since forever.
He had bladder issues. He couldn't pee at one stage, so we took him to the vet and he recovered fine. I remember that well years back.
But then last year he had the same issue. One day he seemed fine, then he started acting odd, and he kept meowing strangely. He threw up overnight and we decided to take him to the vet even though it was the weekend.
I remember spending time with him before he was to go. Everyone had a bad feeling. I sat with him and made the most of it just in case. The last I saw was him going into a box and being driven away.
He couldn't pee again, but this time it was bad. It led to Kidney failure, and blood poisoning. It took hours to find this out, it was awful.
We had the option of surgery, but his blood had become so toxic that he would have almost certainly died during the operation, or be stuck in intensive care afterwards. We spent money exhausting options, but he was around 17 - 18 as well. We couldn't bare to put him through more medical stress just to end up dying, so we had him put to sleep as well.
I cried about it. I was really hurt by it, made me depressed for a couple of weeks and I got terrible anxiety afterwards about silly things. And I don't mind saying that even writing this has upset me, thinking about it again. You never realize how much pets mean to you until they go. And after 10 years, it was a shock that he died. I'll never forget about that little furball, the best mannered cat I've ever seen. He died an old man really, but I just regret that it wasn't a nicer passing.