500 words is very short but remember its a fairly level playing field. Everyone else has to keep their essays to 500 words too. If you write it well, having it be extremely short actually helps in a way. There are going to be those egomaniacs who write 1500 because they feel they are so awesome they deserve the space. Then there will be people who write 500 words and its completely choppy with no flow.
The first few essays are always tough, after you write half a dozen, you get a much better feel for how to work it. I start writing what I want, typically its 2-3 times too long. I then immediately cut out stuff that adds nothing to my position. So that takes out a bunch, I then eliminate stuff you dont NEED. A nice intro is good but you are better off getting right to it with minimal lead in. I then try to combine sentences and make it as concise as possible.
In the end usually I was 10-20+ over. I think ask my first reviewer what I need to get rid of. Take out that last bit and its usually the right length or close to it.
I tended to have 5 or 6 revisions before I even showed it to one person. I am of the mind if you are asking someone to take their valuable time to help you that you should be very well on your way. If you can cut anymore send it to someone and tell them to cut what they think adds the least to your positioning. They wont be attached to anything so they can easily say that your story about X while its good adds no value to your essay. My brother was my first checker, pretty much said what to get rid of and what to strengthen during a quick read for material.