Please dont give up yet and dont despair. There are lots of ways to get through this with time and patience.
Alife, our golden boy, has been seen by two vets who both thought his agression was just behavioural and recommended rehoming etc. But the second vet gave us a month's course of pain killers and after a few days we began to see a real difference. Over the last 3 weeks, he has become more and more relaxed and less growly and snappy, so at least now we know that the "mild" arthritis in his hips is obviously causing him a lot more pain than the vet believed.
If you find the pain killers dont help, then make sure your vet is doing very thorough thyroid testing, as even slight fluctuations can cause agression problems in dogs (but not many vets seem to be aware of this!)
Failing that, find a GOOD behaviourist (APDT registered or similar) to work with you, and also maybe consider some alternative treatment, such as t-touch or bowen technique which can really help stressed dogs.