I'm fairly good with the technical aspect of the English language considering the amount of teaching I've had in it (nothing other than the basics) and the fact that it's my second language. I fall down when it comes to prose and dialogue, though. That's not to say that I can't put together a paragraph or short story that figuratively rocks your socks off, but it's rare. I can also put together some interesting characters, if given time and inspiration, and also know how to avoid clichès in plot, if not necessarily how to always write a good one.
In regards to non-creative writing - reviews, essays, and the like - while I haven't written one in a good while, I'm constantly judging and evaluating, so at least I have the thinking process related to it well under control. But when I recently tried to write a review for a certain very popular game, I simply choked up and couldn't start it off. Anything I typed was terrible and cheesy.