I have yet to master the large form. More specifically, I've yet to manage a large form successfully without bending to the age-old dogma of "goal-oriented music"—that is, I haven't been able to write a long piece while staying true to my current aesthetic concern of writing strong music that is not interested in "going anywhere." It's something which requires a lot of experimentation with musical time and the handling of material in such a way that no true development is perceived in the traditional sense; very difficult.
I also have significant difficulty writing for winds and brass.