Wow...

That diagnostic test at the beginning of the OG is notoriously difficult. There are some great questions there, but I have no idea why they made it a diagnostic - maybe to scare people into studying. Many of those math questions are heavily knowledge-dependent, which makes them very difficult to answer unless you've done some study or you have a very good recollection of the math you learned earlier in life. Most people starting their GMAT prep haven't done math in years, and most find that diagnostic a daunting experience. It's a very good test to do, but only after you've done a review of the material - I'd suggest looking at it again later.

The GMAT is a challenging test, no question, but it gets easier if you work at it. You're not alone in finding the questions tricky at first.

First: take a deep breath. You got a taste of what you're needing to wrestle with, and you'll get the skills, however you study, to improve your score, and move away from the 90% guessing.

There are many, MANY people who have had your experience, so you're not alone.

I recommend the following:

Take a REAL diagnostic CAT test. You can find these online on the GMAC site. While there are alot of third party tests available, and these are great for individual companies and have some merits for the study process, taking a diagnostic by the test-writers is the closest you'll come to the real questions and 'vibe' of the test: how the answers are delivered, how the questions are asked etc. This will help you develop a sense of the patterns on the test, as well as bolster your intuitive self, both of which will make you a better test-taker, help you know directly what to expect, and if necessary, and help you to make intelligent guesses when necessary.

Good Luck and Happy New Year!