Sorry to hear the bad news, Navi88. It's never fun to receive such an unpleasant surprise on test day.
It looks like you've handled your preparations well for the most part, and I doubt that your
MGMAT or GMATPrep scores are terribly inflated. The only thing that worries me a little bit is that
OG 10 overlaps the GMATPrep somewhat, and if you've obsessively studied the
OG 10 questions, you'll probably get a little boost in your GMATPrep score as a result. Even if you don't remember the questions consciously, you'll probably be able to save some time because you can read the "repeat" questions quickly... and you'll have more time for everything else.
Still, all of your
MGMAT and GMATPrep scores fell within a similar range. So maybe that last 38 on the GMATPrep was on the high side, and maybe a 35 or even a 30 wouldn't have been unreasonable result on your actual test. But a 20? That's quite a jump.
I have two related theories, and they might both be wrong.
First: you probably made just enough careless errors early in the verbal section to keep your score painfully low. As you probably know after all of your hard work, your GMAT score depends primarily on the difficulty level of the questions you get right or wrong--not necessarily on how many questions you got right. If you make a couple of careless errors early in the test, you'll see more easy questions, and you'll have to go on a pretty hot streak to climb into more difficult questions. Once you've made some mistakes early in the test, your margin for error is substantially reduced, and even an occasional error (say, every four or five questions) can keep you pinned down to the easier questions. (And yes, I'm stealing from my response to another suffering test-taker on this forum:
540-no-idea-what-went-wrong-142892.html)
So you didn't necessarily have to miss very many questions to get that 20 on verbal. If you missed easy questions in bad spots, then it's not unreasonable to have this sort of score meltdown. It's not a comforting explanation, though.
Theory #2 is even uglier. You mentioned that you felt perfectly calm during the test, but sometimes test-takers internalize their stress. The sick, twisted thing about the GMAT (both quant and verbal) is that you only need to mis-read one key word on any given question, and you'll end up missing the entire question. It doesn't take much nervousness (or fatigue, for that matter) to cause you to make some errors that you don't normally make. And again, if those errors are badly placed, your score can suffer a huge dropoff... even if you don't make very many of those errors. Sometimes, the test magnifies your nervousness. It's not necessarily fair, but that's how the GMAT works sometimes.
I obviously don't know you well enough to even begin to guess how well these theories fit, but they probably explain at least some of the story... albeit in a completely unsatisfying way.
It's probably not reassuring when the test-day agony is still so fresh in your mind, but these sorts of things happen on the GMAT. You'll run across plenty of others on GMAT Club who have had similar experiences. Don't let one bad test get you down--obviously, you have the ability to score substantially higher!
As far as materials go, you might consider spending some quality time with retired LSAT tests for RC and CR practice. The language is incredibly difficult, and the LSAT tests your ability to read precisely, just as the GMAT does. You'll notice plenty of cosmetic differences in the questions between the two tests, but the LSAT will definitely improve your ability to read hard passages and focus on the nuances of those passages. Hopefully, the GMAT RC and CR will feel relatively easy after you've suffered through enough LSAT questions.
I didn't see any mention of the GMATPrep Question Pack (available through the GMATPrep software) or the GMAT Verbal supplement in your post, so you'll want to do some work in those if you haven't already. And
OG 13 does have some new questions that weren't in
OG 12, so it *might* be worth the price to go through those questions, too. (A full list of the new questions is available on the
MGMAT site.) So don't worry, there are still plenty of resources out there.
Good luck with everything!