I had some similar problems at first with SC, but once I began to not worry about time my performance level jumped up. Part of my problem at first was I felt like I needed to rush everything, but the reality is you have plenty of time to read the sentence and determine an answer as long as you simply are thinking of getting an answer rather than answering fast.
MGMAT SC is an excellent book, IMO. I think one area where they fail though is emphasizing the speed (I think they want 1 1/2 minutes max per question) which is fine once you have mastered the concepts, but it is not what you should be doing if you are having problems with SC and are a beginner. It is much more important that you not only understand the concepts but understand how to actually apply them to a GMAT problem, which is also another problem with the book as they really give you many GMAT style examples other than referring to those in the OG.
One of the techniques they discuss early but can probably easily be forgotten by the time you get to the end of the book is utilizing splits to quickly eliminate answers. There are a ton of questions that can be broken down in a 3/2 split and some choices are very obvious. Set your scratchpaper up with columns and as soon as you find those splits immediately cross off those selections so you can focus on the other two/three answer choices. Sometimes I found myself wasting so much time on my practice exams by going back and rereading choices that I never should have even been considering a second time.
The other thing is dont go nuts trying to find everything wrong or right with a sentence. All you have to do is find one thing wrong with a sentence. As Paes said in the post above, its easier to eliminate than search for the correct answer. In many cases there is something pretty blatant wrong with one or two choices. If the subject and verb dont agree its wrong. You dont need to go any further than that. Once you know its wrong cross it off your list and dont look back.
Without much time to prepare, if I were you I would go back to the OG(assuming you really do have the concepts down) and start from the beginning. Do 25 questions at a time. Dont go crazy thinking about time, but note it. My guess is you will end up not far off the mark. After you finish your 25 questions check the answers. Review what you did wrong. The explanations in the OG are not great, but you can always post the questions here and get a very detailed reply if you need more clarification. If you have alot wrong early on you probably need to review the SC guide again. If you did well move on and do another 25 questions. Repeat the same checking process. At most that should take 3 days to finish. After that take your MGMAT question bank under timed conditions and see how you do. If you are doing well enough take one last practice test and then decide if you really want to take the GMAT now or instead reschedule it.
