I feel your pain.
I scored a 530 a few years back without studying, then 590 recently w/AWA 6.0 after 1.5 months of practice.
I score 640 now in practice tests, on my way to 690. The GMAT is not really an intelligence test--for many reasons. however, it does test elements of 'knowldege' (math, language etc.) and critical thinking. You are probably used to being a top performer in many things academic (I am the same) but the GMAT kicks your @ss--especially if you have a humanities background! Well, you must practice and practice some more! Many of the people who see huge jumps in scores know the GMAT inside and out, all the tricks, question types, common errors and subtle differences between GMAT questions and other types of questions. This level of familiarity is important.
Give yourself at least 4 months of intense practice. Do as many questions as you can find and analyze your results. Cover as wide a scope of types of GMAT questions as you can also (especially in quant). Take many practice tests, but limit them to GMATPrep (take as many times as you can), Power Prep, and PR (this is ok). In my opinion, master the OG 11 and supplements.
I look at it as practice, not study. That frame of mind helps me focuse on the test as a "performance" rather than a test of intelligence. Obviously, the forum is a great place to see a huge range of questions all with very clear answers and explanations from some true GMAT savants.
cheers.