@snood,
That's very tough. You have my sincere sympathies.
A lot of people can't understand how some of us can become so emotionally tied to "just an animal."
When my dog died a couple of years back I blubbered like a baby and I still miss her - truth be told, more than my mother because Nala was infinitely more loving than my severely damaged mom.
In any case, the only advise I can offer is based on what we went through.
We knew Nala was growing old and it was painful for her to move around, but we didn't know how bad it was. When she had a devastating episode, my wife and daughter didn't want to take her to the Vet because they feared he would say we needed to put her to sleep.
We did take her and found she was riddled with cancer in her bones and lungs. The Vet said she never saw such a bad case and was amazed Nala had lasted this long (I should point out that this was a new Vet and not the hacks who missed her horrible condition).
Clearly there was no option but to put her to sleep.
Your cat may not be in as horrible a position as my dog was (I hope for his sake this is the case), but an animal that is constantly losing weight is ill. If your cat is not eating that is a very serious sign.
We're fortunate enough to have been able to afford all of the tests. Unfortunately our regular Vet never recommended any of them
I can understand how someone may not be able to afford these tests because they are certainly not cheap.
You are in a terrible position: Do you hope for the best and unintentionally allow your beloved pet to suffer or do you take a step that is utterly final? I don't envy you.
The only thing that I would throw out to you is that animals have a much higher threshold for pain than humans and their reaction to it is much different than ours.
Do some research and see how cat's express pain. (For some dogs, it is excessive panting). If you see the signs, you have to make a tough choice, but,ultimately, I'm sure you would rather your pet die before his time than suffer terribly.
All the best with this.