This is a hard question for me because the only thing I find HARD about writing is not being able to write fast enough. I like telling the story, and creating scenes, and surprising myself when a plot comes out of the blue.
If I had to pick one thing about writing that makes me turn off the computer and take a break before I give up writing entirely... it would have to be grammar.
What about you? If you're a writer, what do you have trouble with and would like to master?