apologies if this has been mentioned already. I was too saddened by the original post, to read all the replies.
I'm surprised that the vet hasn't recommended another opinion, perhaps by a specialist (oncologist?), if they can't seem to identify the problem.
Having been down this road 2x last year with kitties (one 13.5 y.o., the other 9), and like everyone else here, I feel for you.
I firmly believe that our pets communicate with us, if we're willing to listen. I struggled with what to do with the 13.5 y.o., and she told me when it was time. Never regretted moving forward with euthanasia for one single second after that. While it was one of the hardest things I've ever done, I did it for her and it was absolutely the right thing, at the right time.
Special hugs for you.....