Generally speaking, if you don't like your score on your first test, then you likely need to re-use the materials you have already been using, because you didn't learn everything that you needed to the first time around. Getting better is not about doing a bunch of new problems. It's about thoroughly analyzing the best problems you can get your hands on (
OG, of course) so that you recognize what to do when you see something similar on the test. The one major difference is that you (hopefully) have a better sense of your strengths and weaknesses going into a re-take, so you can prioritize your study accordingly.
If you took the course, go get the hand-out from class 2 (you can download it from the Course Downloads page in your student center). That hand-out gives you 10 questions to ask yourself when reviewing problems. Have you been using these? If not, start doing it - this is how you thoroughly analyze the problems you're doing. Ask yourself these questions on every problem you do, including the ones you get right. In a 2-hour study period, spend about 30-40 minutes doing problems and the rest of the time reviewing. (Most of your learning comes from the review and analysis you do after trying the problem!)
FYI: the average score improvement from official test 1 to official test 2 is about 30 points. (That's for ALL test takers, not just
MGMAT test takers.) So, it's some improvement, but not huge - not really statistically significant, actually.
Also, practice tests are very good for figuring out what your current scoring level is, but you don't get a lot better merely by taking a practice test. So consider saving one of those GMATPrep tests. Alternatively, You can still take both GMATPrep and
MGMAT CATs with repeats as long as you follow a few guidelines to minimize the chance of artificially inflating your score via question repeats. First, anytime you see a problem that you remember (and this means: I know the answer or I'm pretty sure I remember the answer, not just "hmm, this looks vaguely familiar..."), immediately look at the timer and make yourself sit there for the full length of time for that question type. This way, you don't artificially give yourself more time than you should have. Second, think about whether you got this problem right the last time. If you did, get it right again this time. If you didn't, get it wrong again. If you *completely honestly* think that you would get it right this time around if it were a new question (even though you got it wrong last time) because you've studied that area and improved, then get it right this time.
Good luck - let us know how it goes!