You did all the hard work preparing for GMAT including:
+ getting advice from your smart ass MBA friend or colleague
+ preparing from the best books money could buy
+ taking expensive online tuition courses that promised the world
- didn't get the Quant score you wanted
Why? It boils down to the pyramid below.
Most of the students and tuitions focus on Knowledge. Although important, it only serves as the foundation. Unless you up-skill in other areas of the pyramid, you would struggle to score well on GMAT. Let me explain what each area requires and how it can help you achieve your target score.
This is the syllabus and includes the basic theory behind all GMAT topics. To master this you will have to use the right books and resources e.g. Official Guide is a great starting point. Try to use flashcards to memorise the formulas and theory for each topic.
Knowing the basics would not be sufficient. Each topic has special tricks and techniques that you should learn from a GMAT guru. Most of the times these techniques are not condensed in books but if you want to go at it solo then feel free to use quality GMAT forums e.g. Gmatclub.com, Magoosh, etc
This is the level where your exam taking skills kick in. It is all about being clear about the question being asked and the topic the question is related to. This is where you bring your knowledge into action by interpreting the question right. In almost all cases, students struggle with this the most. Best way to master this skill is to use simple examples to decipher what is being asked.
If you approach the problem in a structured way then your chances of getting it right improve significantly. This includes habits like boxing your question and using visuals to represent the question.
Break down the problem into smaller chunks. Focus on the small chunk and solve that first. Then move to the next part. Link the solutions as you weave through your final solution but always focusing on the question box mentioned above. This also includes time-management, decision-making and guessing but I will have to write a whole new topic to explain that.
Exam stamina is analogous to marathon stamina. Try to sit yourself in timed exam conditions. Start with 30 minutes fully focussed time slots and gradually improve your stamina to exam readiness. If this is too much or too early for you then at least time every question you do. In that way you will introduce the element of stress which will you can learn how to overcome.