Hey there,
I agree that you may have underestimated the preparation needed for the GMAT. Can you describe how you studied and what you used?
To provide context with my study schedule, I studied for a total of 2 months. I took a practice test on Day 1 to get a baseline, which was a 600. I used the official GMAC guide and the entire
MGMAT series, and also signed up for
Magoosh.com for access to their practice questions and video tutorials. Every weekday, I studied for 1.5 hours in the morning before work, and then another 45 minutes in the evening reviewing what I had learned. On Saturdays, I studied for a solid 3-4 hours (preparing myself for practice exam stamina). On Sundays I rested.
Starting month 2, I replaced my Saturday study sessions with full-length practice exams. I was averaging scores of 680-690 during my practice exams. On test day I ended up with a 660 because I lost time in quant due to palm reader delays (make sure to account for this!!).
Scores in the 400's signal that you have not yet mastered the basic concepts tested on the GMAT, or that you are falling for a lot of traps. I think going from a 400 to the 600's should be very doable though! Questions in this range are really just testing the basic concepts. Give yourself at least 2 months, and make sure you thoroughly read through the
MGMAT books for the Quant section and master all the basic concepts, most importantly in
arithmetic and
number properties. More advanced questions will assume that you have a sound understanding of these concepts. For verbal, make sure you master parallel structure, tenses, and the most frequently seen idioms (
MGMAT SC is excellent). If you master just these concepts, you should find yourself in at least the high 500's to 600. Just do lots and lots of practice questions, and review all of your wrong answers to understand what you did wrong. Keep a wrong answer log, and try redoing wrong questions after a week to make sure you've actually learned the concept. Time everything you do - you should try not to go over 2 minutes with any one question.
You should also make sure you have solid test-taking and guessing strategies. There are many traps on the GMAT, so understanding how to leverage different strategies to your favor is crucial. For example - question timing, strategic guessing, picking numbers, and using logic to eliminate wrong answers. Knowing when and how to guess is important as well - i.e. if you are running behind on time and come across a very difficult question. It's better to guess (strategically!) on a very difficult question that you will likely get wrong to gain valuable time that you can use on the following questions.
If you're a visual learner,
Magoosh.com has video tutorials for their lectures as well as for all of their practice questions.
Hope this helps!