Pavan my friend, I'm in your boat as well.
I just keep practicing them and everytime I miss a problem I note down the concept that was tested.
In the end, I have a tally of what I missed the most and what I missed the least and all the 'in betweens'. Then, I just refer back to the MGMAT SC
guide AGAIN and review the concepts and practice them more.
I understand it can be frustrating but to keep practicing is all we can do!!
But DO, make sure understand that when you test yourself, and mark the four choices incorrect, you understand WHY you're marking them incorrect. This is something that helped me out!
And, of course my favorite- IDIOMS!! I'm a native speaker but there are still lots of Idioms which I have to memorize. SO, needless to say, make sure you do the same.