After you have gone through the essay thoroughly, and have eliminated all the possible errors, spend some time away from it. This should give your mind a break, which is necessary to find mistakes which may go unnoticed if you read the essay immediately after you write it. If you are still not satisfied with it, get a second opinion. The neutral criticism may be tough to accept, but it will help you improve better than even the harshest evaluation you can do of your own work.