You found an abandoned dog and wanted to keep it. You gave it food and water. You took it to the vet. You've tried everything but the dog is still sad. It's time to make it happy and healthy by building trust.
Feed it everyday. Give it its favorite food mixed with your dinner scraps and water. If your dog doesn't like this try making diffrent combinations.
Get it a friend. Once you and your dogs are nearly soul mates you might want to take getting another dog into consideration. If you cannot handle it, take it out for more walks so it can meet other dogs.