run (one's) head into a brick wall

To attempt, continually and fruitlessly, to accomplish some task or achieve some goal that is or seems ultimately hopeless. I feel like I've been running my head into 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 running your head into a brick wall trying to convince everyone.

run one's head against (into) a brick/stone wall, to

To make vain efforts against insurmountable difficulties. This expression, with its vivid image of futility, dates from the sixteenth century, when John Lyly (or some other author) wrote, “Thou shalt . . . have thy head runne against a stone wall” (1589). Other equally hard objects, like doors, were cited in similar fashion. By 1887, however, it was “If we run our heads against walls we’re safe to hurt ourselves” (M. Sergeant, Jacobi’s Wife, 1.1).