If you read the many experiences posted on this forum you will quickly realize that your situation is quite common. You will also realize that many people have improved their scores dramatically by their second, third or even fourth attempt. Think of it as a challenge, a game if you will, one that you will eventually win.
You said that you had problems with basic concepts. In that case, I would focus much attention on the first 2/3 of
the Official Guide, especially the first 1/3. At the same time, I would encourage you to become an active participant in this forum, especially the math section. By sharing your ideas, getting feedback, and answering questions, you will not only develop a comprehensive skill set, but you will also gain a sense of confidence - confidence that you can use to attack harder math problems.
I wonder what your GMAT prep scores were? Remember, GMAT prep is by far the most reliable predictor of your 'true' performance. Also, if you delete the exam folder in the GMAT directory and reinstall, choosing repair option, you will gain many new questions. Thus, you can use GMAT prep on more than two occasions, and still get a good estimate of your ability. I would suggest that you go over the basics first. After you're done, take a GMAT prep exam and see how you do.
Also, you have not provided a list of study materials. However, for basic concepts, books such as Princeton Review and Kaplan do a fairly good job. But do not forget
the Official Guide (by far the best book). For sentence correction, I highly recommend (and most other members here), the Manhattan Sentence Correction Guide. This book is excellent. It uses questions from
the official guide to teach you the concepts neccessary to master the SC section. For Reading Comp, I would suggest that you get hold of good quality publications, such as Time, Newsweek, Economist, and start reading them on a regular basis. For CR, I suggest lots of practice with an emphasis on identifying the patterns that underlie CR questions.