bang (one's) head against a stone 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 banging my head against a stone wall trying to understand this math equation. Some people are never going to agree with you on this, so it's no use banging your head against a stone wall to try and convince everyone. I'm banging my head against a stone wall trying to get funding for my project. So far, I've made only $20.