Hi chinmoi,
From what you have described, your biggest 'issue' is that you do too much work 'in your head' - and you should NEVER allow yourself to work that way. Taking the proper notes helps you to stay organized AND work faster. Thus, the immediate area to focus on is your 'mechanics.' Here's a simple way to measure whether you're taking the proper notes or not: after solving a GMAT question, if you handed your work pad to another person, would that person understand all of the information that you were given in the prompt, the question that you were trying to answer and the 'steps' that you had worked through? If the answer to any of those questions is 'no', then you have NOT taken the proper notes. There could be additional issues that you need to work on, but you really need to 'fix' this issue first (as it applies to every section of the Exam).
Yes - revisiting questions that you got wrong (so that you can reattempt them step-by-step on the PAD) is a worthwhile use of your time. In addition, you have to consider more than just the questions that you got wrong. Which questions did you 'guess' on and get correct? Which Verbal questions did you 'narrow down to 2 choices and "guess correctly"?' How about the questions that you got correct but you spent a LOT of time on - could you have used a different approach and saved time?
GMAT assassins aren't born, they're made,
Rich