Don't get down on yourself. Be glad it was just your essay and not your GPA. The essay is the easiest to improve (and you don't have to spend more money on re-taking classes).
I had spent about a month working on my essay. 100 words is not easy to express yourself. I think that I had a hard time in the beginning because I wanted to say so much. I read and re-read the question constantly because at the end of the day, even if you want to include personal stuff in the essay, you MUST answer the question directly.
I think it's great that they said everything else was fine! You can improve your essay - I know you can. Ask for help from a professor, friend, etc. A simple switch of a word can make a big difference. If you need help, I am here too!