knock head against a brick wall
knock (one's) head against a (brick) wall
To attempt continuously and fruitlessly to accomplish some task or achieve some goal that is or seems ultimately hopeless. I feel like I've been knocking my head against a brick wall trying to understand this math equation. Some people are never going to agree with you on this, so it's no use knocking your head against a wall trying to convince everyone.
knock one's head (up) against a brick wall
Fig. to be totally frustrated. Trying to get a raise around here is like knocking your head up against a brick wall. No need to knock your head against a brick wall over this problem.