It can be crazy-making to apply time and effort to something that doesn't yield. Ugh, sorry.
I'm going to add my voice to the chorus about the MGMAT SC
book (and not just because they're my employers!); I've seen huge improvements when people are willing to work through that volume with patience and attention. You must also be willing to cross-train between reading grammar rules, practicing grammar, doing OG questions, and reviewing OG questions you've already done...all four steps are important.
Ideally, students will work through OG questions in conjunction with grammar content work and systematically review and learn from that review process. When you say you've gone through the OG, what do you mean exactly? Do you understand the explanations when you read them? And do you have a repeatable SC process that you execute on these types of questions? If you feel like you're totally lost then a tutor may be the way to go--if SC is your only weak area, then you may not need that much time at all-- just someone to point you in the right direction with a repeatable process, then send you off to study grammar, and do brush-ups as needed with work in the OG or grammar rules you don't understand.
Good luck! Don't lose heart-- this stuff really does yield to effort...it's finding the right kind of effort that's the trick.
JP Park | Manhattan GMAT Instructor | Los Angeles
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