I put my cat down last year at 16. He was throwing up weekly - not - I ate my food too quickly - It was like bile. I went to my vet but they were totally unhelpful. Always wanting to do a 2K treatment / test (and I spend 2K trying to get a diagnosis). I never got a diagnosis (I suspected lymphoma) In the end my cat just slept all day when he wasn't throwing up and I knew that was no kind of life. Everyone told me I could give him 2 more years... but I was like... forwhat? I do not even know if he is suffering. My gut was telling me it was time.
It is difficult to make that decision, I probably delayed for 6 months or so, but in the end I my gut was screaming at me that it was time.