fight a losing battle
To try persistently and with great effort to do or achieve something that is ultimately doomed to fail. You're fighting a losing battle if you think you can convince Sarah to go to college. It looks like I'm fighting a losing battle trying to get a raise from my boss.
fight a ˌlosing ˈbattle
try without success to achieve or prevent something: I’m fighting a losing battle with my weight. I can’t lose any.The police are fighting a losing battle against car theft.