Sorry to hear that your score has not improved at all after so much effort.
I have not taken GMAT yet, but since I have started my prep and took few practice tests, I think I can put some comments to help you out.
You have mentioned that you have finished studying ... books. Did your scores while practicing improve along the way? If the answer is 'No', I feel this is the loophole in your preparation; if your answer is 'Yes', ask yourself why that improvement s not reflected in actual GMAT even after many attempts.
I have some simple strategy here:
1. Test:
Knowing theory alone will not help to score on GMAT, rather it would help in academic exams. What matters is do you apply it well on GMAT? Moreover, sometimes we feel that we know theory but actually we do not. So, you need to test yourself first whether you know all the fundamentals or not. If you are answering many easy questions incorrect, it means that you need to start from basics. There are many books which can help in improving basics. But since you told that you have finished all the
MGMAT guides, if you feel that these books can help you, start studying them again and test yourself along the way; otherwise, online/offline tutoring can help you.
2. Practice:
Practice according to the test results. 5 topics-5 days is my strategy to practice.
3. Improve:
This might be the step you have missed in your strategy. You might have practiced a lot, but if you do not improve your skills along the way, your score is also not going to improve. Have you analysed each and every question you have visited as part of practice. If you do not analyse, you may answer a similar question incorrectly again in the future. So, you should analyse and keep an
error log so that you can test yourself again using the questions which you have incorrectly answered previously.
4. Repeat steps 1, 2 and 3.
I hope this helps.
Remember two thing:
1. If GMAT is not a test for you, try GRE
2. Life is not about GMAT, we are great people and have been doing great things so far. Try to peruse what suits you best.
Regards
Nikhil