Hi!
1.) In regards to Verbal:
If you feel you need more grammar practice we would recommend "English Grammar in Use" (intermediate and advanced levels).
Please be aware that almost all grammar books tend to be exercise-oriented and often go into much more detail than what the GMAT tests. In addition, many grammar books do not deal with certain sentence structure issues (e.g. dangling modifiers). You need to look for something that can help in that regard too, or simply fill that gap with a proper course such as ours. You need to cross-reference what is on the GMAT with the book you get. But "Grammar in Use" is probably the best grammar self-study book available.
2.) In regards to Quantitative:
If you find that you experience serious difficulties, you may want to supplement the learning with independent study of key concepts. No program teaches math from scratch; it would take about 6 months to a year of just math studies. Also, remember that part of achieving success on this exam has to do with strengthening your weaknesses AND your strengths. The section you are strong at will help you with your overall score, too. Remember that you will need to learn how to attack the GMAT Quant via various techniques aimed at saving time,and NOT solving everything fully. You can certainly do a bit of work on the side in math to strengthen your skills, as this cannot hurt.
3.) In regards to pacing and timing:
It is fine to hesitate momentarily to think about what you are looking at, and to ask yourself "What have I learned about this? What traps can there be? What am I supposed to check?" AND to remind yourself: "I will not read the text or the answer choices too quickly."
Essentially, when you look at a question, such as Subject Verb Agreement, when you see a verb you should ask yourself where the subject is and what type of subject it is. By doing this, your mind will start running through all the different subject types there might be. Then you will ask yourself about the tense: "What is the tense used? Is it the right tense? Is there an indication that it should not be this tense?" etc.
For most people, 10%-15% of their errors are avoidable. Eventually, once the knowledge is in you and you know what to check without asking yourself the same questions each time, you will be able to notice rather than have to pause. But for now, train yourself to read carefully, to identify the question properly, and to notice the small differences among answer choices. (For example, why do "b" and "c" both sound good, and what is the difference between them-- because one is wrong?)
A slight pause and self-reminder to slow down may help you. It is OK to be pressed for time, however, looking at the clock after every question will not help. Nor does it help to rush through things. For now, simply try to remind yourself of what knowledge you are supposed to apply, and what you are supposed to check/identify. Focus on attacking the questions in as systematic manner as possible.
Hope this helps!
Sincerely,
Evan