Really feel for you i do. Helpless doesn't even come close, does it.
Vets are not infallible and do not always know the answer, must be a hard job to do.
The bitch i mentioned, was found to have a mass later on, nothing picked up with tests prior.
I'm lucky that i have a good relationship with my vet, and he sorta talked me into letting her go, something i would not have thought about myself as she was THE dog. She went at home eating best steak mince from my hand, oblivious to anything the vet was doing, the last thing on her mind was the gob full of steak with more to come.....a brilli