I can kinda relate as I have a sick dog as well, he has a back injury which causes him to think 1 flea is a thousand and he goes after it. It certainly plays on ones nerves to try everything and have nothing work for more than a few months. He also luvs to go out and hunt with the other dogs, all I can do is keep trying to give him the best life I can. IDK when it will get better, but I hang in there for him.