I majored in English Literature with an emphasis on writing. I've written more essays, papers, and creative works than most people I know. Still, the hardest piece of writing I've ever attempted has been the Statement of Purpose for grad school. If you're struggling on writing this, here are a few tips to help:
1. Read Directions - Each school has different requirements for this essay, so make sure you read the directions carefully. If they ask for 500 words, don't ramble on for 1,000 or fall short at 300. This may sound like common sense, but which all of the additional stress of grad school admissions, many prospective students pawn off one essay for all of their applications, and easily get ruled out for failing to read directions.
2. Get Help - Once you have your final draft, show it to people you trust and respect. Ask for their opinions and listen to them. You're allowed to get feedback, so use this to your advantage.
3. Stuck for Inspiration - This is the most common problem. Try giving yourself 10 minutes to write and just keep going. Even if what you produce isn't winning material, you may get a good thought or direction out of this exercise.