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.
Give it some freedom. But never too much. Until you build full trust with your dog only let it out for a walk everyday and maybe a little peek though the garden.
Start playing games with it. After about a month your dog should start to trust you. Play football, catch, races and other fun things.
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.