I have an idea, and it seemed like I had a somewhat clear understanding of the story, it's theme and most of the major plot points, but no matter how long I sit there and how drastically I tweak the initial idea, I can't construct an outline. I tried to do it as formuliac as possible, working based on beat sheets, filling in major plot points and then trying to get smaller ones, but it gets me nowhere. I also tried to just go with the flow and written couple of free plot summaries without thinking about structure, but the result is too terrible. I have a deadline in two days and I don't know what else to try.
I'd guess the deadline pressure is blanking your mind.
What I would do is this:
You have everything in your mind. So stop thinking on it. It's time to live the story. Pick any scene or character or phrase that you feel anything about and take a day to write down the whole story from there. Allow yourself to daydream your tale and explore what it means for you and where it takes you.
On the second day, take what you have written and distill this into an outline. Add what you think is missing in a few brief words, but otherwise just extract the structure of your "daydream".
Also, drink enough water (about a small glass each hour), get enough sleep (sleep deprivation makes stupid), move your body regularly (walk some stairs for five minutes or run around the block, its been shown to increase mental performance drastically) and stop procrastinating by surfing the web or reading how to books (you know all that already).
Good luck and tell us how it went.