DS? I've figured out all the other abbreviations, but that one is eluding me.
My experience with the Kaplan practice tests was that their scores were approximately 100 points less than other practice tests (and therefore, the GMAT itself). So... let that be a boost to your confidence.
Also, my understanding (i.e. I read it in one of the prep books, but can't remember which to properly cite it as "fact".) is that the probability and permutation questions are high level. If you're getting them, you must be doing well. Again... a confidence booster.
Personally, I REALLY benefitted from Princeton Review's "Cracking the GMAT". I thought it did a much better job than Kaplan in explaining the basic strategies for approaching each question. Kaplan's GMAT Premier was very useful to me for boning up on my math and (OK; I just figured out for what DS stands) and data sufficiency questions.
I seem to recall that Kaplan was a bit weaker on the verbal questions. SC and RC gave me trouble initially, but PR gave some really good tips on RC. (I'm looking back at the PR book right now, at the things I underlined, and PR's comments really do agree with what I encountered on the test.) Yeah; VERY good tips from PR.
Timing-wise: you don't say whether you're having trouble finishing the Q or the V sections. Using scrap paper in the V section to mark which answers were definitely wrong - as opposed to having to waste time re-reading each answer choice to remind myself - was an incredibly useful suggestion that brought my V score up significantly.
I know this is fairly general (I could write an essay on what I did to succeed on the GMAT, but it'd be quite lengthy), but I hope this helps even a little bit!