Thanks for your replies so far, I've found them very helpful. I feel I can write a decent short piece and that there's nothing really wrong with that, it's just that when I listen to someone like Beethoven I'm struck with deep admiration of how he just keeps elaborating; long after most composers would have ended their piece he goes the extra mile, and I think that's what makes his music so special. His music is just so epic, and takes me to a whole other world, something which is really hard to do in only two or three minutes. Anyway, I think I'll try planning ahead a little more, I usually just start writing without giving too much thought to the structure until I'm a quarter of the way into it, perhaps that's my problem.
Thanks again,
Jackson