A hard topic does not tell you that you cannot learn it. It may tell you that the current explanation, pace or practice method is not working. After an attempt, look for a specific error, ask for feedback and try a different strategy. For a calculation, that could mean explaining each step aloud before repeating more problems.
Progress is easier to see when you check understanding instead of counting hours. A practice test or brief explanation from memory can reveal gaps that rereading may hide.