I'm sorry that you had a frustrating experience on the test. You're not alone; a lot of people struggle with this test.
Asking how to start preparing again isn't quite the right question / direction. First, you need to figure out what went wrong the first time around. It isn't the case that you necessarily need all new material; your approach the first time might not have been optimal. That is, you may not have learned in the way that you needed to learn in order to extract the maximal amount of learning from your material. It may not be about WHAT you studied; it's more likely about HOW you studied.
You mentioned that your score on the real test dropped, though you don't say by how much exactly (the best I can estimate is more than 50 points). If your score dropped 80+ points, then you can use this article to start trying to diagnose what went wrong:
http://www.beatthegmat.com/a/2009/10/26 ... went-wrongWe also need some additional data about your strengths and weaknesses. If you've taken an MGMAT CAT, you can use this article to analyze your last test:
http://www.beatthegmat.com/a/2009/09/23 ... tice-testsCome back here and share what you discover and we'll help you figure out what to do.