Hi Sanjoo,
I am no GMAT expert, nor do have I taken the test yet.
But below are some advice from my side.
1. Take some break, relax for few weeks, do what ever you like to recharge your batteries to the fullest. This is not the end of the world.
2. Get prepared mentally to take the test again. The test doesn't have limited attempts, because many of us don't do it right at the 1st attempt.
3. Take a diagnostic test again - this time instead of focusing on 5 parts of GMAT(PS,DS, SC, CR, RC), focus on sub-areas such as for PS - Number theory, Combination, Time and Work etc. For SC - S-V agreement, Modifiers, Comparisons, Parallelism etc. Find out your strong and weak sub-areas and work accordingly.
4. For RCs: Make a habit of reading regularly. I would suggest "The New York Times". Read the daily opinion page. Since you are an Indian, I am sure you will find the language and style of writing strikingly different from what we read here - "Times of India" or "Hindu". This will make you familiar with the language used in GMAT RCs. Once you read the opinion, write a quick summary paragraph. This would ensure you comprehend the text properly.
The
GMAT fiction prescribed here is good, but it is time consuming and recommended only if you have 1 year in hand. Reading 1 or 2 non-fiction management books would be good for your knowledge. So it is up to you.
5. For CRs: Consider each as an AWA. Before attacking the question, find out what are the possible assumptions, given facts of the argument and conclusions. Then read the question and mark your answer.
Generally people read the question 1st, but that is recommended once you are apt in the above strategy.
6. For SCs: you need to : understand basic rules , practice a lot and revisit error logs.
7. For PS and DS: Number Theory, Powers, Ratio, Time and Work, Percentages, Interests, Mixtures - Buy Quick Arithmetic by Asish Agarwal - S Chand publications. This is really a very good book, which provides some tricks and fundamentals very useful in GMAT. For : Sets, Inequalities, Statistics and Probability - I have not found any basics book yet. But while practicing problems you use google and note down solution methods.
I am sure, you have done most of the materials. So you definitely need a break to erase all the wrong concepts and to start all over again.
And get a job. I think by studying 10-12 hrs daily, you have over-prepared and this fatigue led to your low score.
All the best.
Don't loose hope.