Hi bbednar,
You're closer to your score goal than you probably realize, but if you think that there's something 'wrong' with how you're studying, then you're probably correct. I agree that taking a CAT this weekend would be a good idea - you should make sure to make it 'match up' with Test Day in as many ways as you can (take the FULL CAT - including the Essay and IR sections, take it away from your home, at the same time of day as your Official GMAT, etc.).
GMAC has publicly stated that a Test Taker's Official Score is within +-30 points of actual ability. Assuming a similar 'swing' in how your CATs function, the three scores (570, 540 and 530) you mentioned all revolve around 550 or so. That is likely your current ability level. Raising that to a 610+ in 18 days IS doable, but you'll need to make some adjustments to how you 'see' (and respond to) the Test.
1) On your last CAT, how many questions did you get wrong because of a silly/little mistake?
GMAT assassins aren't born, they're made,
Rich