Allow me to say my personal opinion , but it seems Either you lack basic foundation concepts which GMAT tests Or you get trapped by the GMAT tricks/subtleties (which also have some patterns).
You are the best person to judge you and root out what is holding you !This surely doesn't mean that you should not get any tutors, etc.. but my point is that You are primary, rest is secondary - so be motivated, focused, make use of this wonderful place (gmatclub

) and opt for some tutor course/s (if you feel so) !
Coming to the Verbal matter -
I'd recommend that you must read or revise (in case you have already read ) -
"Foundations of GMAT Verbal" by Manhattan
The Sentence Correction Guide
and then Aristotle Sentence Correction for Sentence Correction
I guess, you are also not so strong at CR, for CR -
The Critical_Reasoning Guide and then Critical Reasoning bible by Powerscore