beat head against the wall
beat (one's) head against a/the wall
To attempt continuously and fruitlessly to accomplish some task or achieve some goal that is or seems ultimately hopeless. Primarily heard in UK, Australia. Some people are never going to agree with you on this, so it's no use beating your head against a wall trying to convince everyone. I feel like I've been beating my head against the wall trying to understand this math equation.
beat one's head against the walland bang one's head against a brick wall
Fig. to waste one's time trying hard to accomplish something that is completely hopeless. You're wasting your time trying to figure this puzzle out. You're just beating your head against the wall. You're banging your head against a brick wall trying to get that dog to behave properly.