It's totally learnable.
For math, just review high school level algebra and geometry, and do a bunch of practice questions (preferably out of a variety of test prep sources--Princeton's the best, then Kaplan).
For verbal, vocabulary is KEY!
My diagnostic test reported my score as a 940 (yikes). I thought I was doomed. After a month of diligent studying and memorization, I ended up getting a 1530 (750 V, 780 Q) on the actual test.
So just study, study, study! It pays off.