When I began studying for the GMAT I believed that I could never get the quant score that I wanted. But with a lot of practice, it can be done.
Most GMAT problems are very very easy once they are broken down. The key is understanding the underlying principles, and practicing those. If you have time I suggest getting a high school Algebra book and going through it. Here you will re-learn a lot of concepts in depth. The MGMAT books
are good, but focus so much on GMAT problems that you might not really get at the underlying principles.
Keep a log of every problem you missed and go over them as often as possible. Eventually you get the hang of it. Do every single OG problem, over and over. Think of the GMAT as a marathon you have to run. You have inside of you everything you need, but without rigorous training, you are not going to make it. Be really consistent with the time you spend on the exam.