be like running (one's) head against a (brick) wall

To be very frustrating and seemingly futile. Trying to understand this math equation is like running my head against a wall. Can you please help me? Some people are never going to agree with you on this, so I'm not surprised that trying to convince everyone is like running your head against a brick wall. I'm trying to get funding for my project, and it's been like running my head against a brick wall. So far, I've made only $20.