Hey Buddy:
Don't loose hope. If you did the test and you only score 320 after three months of study, then you have definitely been focusing on the wrong materials.
Like krismucs said, you have to get the basics down. People learn differently; some people can learn concepts just by reading a book while others are more visual and require an instructor or a classroom setting to succeed. I suggest you check out wingmat.com for a good foundation in Math and the Princeton Review book for Verbal Basics. After you have mastered the foundation, then you proceed to the OG and the GMATPREP ( http://www.mba.com/mba/TaketheGMAT/ToolsToHelpYouPrepare/GMATPrepProducts/FreeGMATPrepTestPreparationSoftwarein2006.htm ) software. After Gmatprep and OG, you should try to get your hands on the SETS.
Still if you are not confident, you can take an online prep course: My recommended course is the http://gmat.grockit.com/