Sentence Correction

I;ve been studying for about 2+ months for the GMAT. I'm taking the test end of Oct. I am really struggling with Sentence Correction. I am in the MGMAT test prep course and have read ths strategy guide cover to cover. I've read the first half of the book twice and I've made flashcards but nothing is sticking..and I for the life of me can't figure out how to improve my score. I'm still at my original score in 50% range right now. If i donti mprove in the next couple of weeks..I dont see myself even breaking 650+. I am a native speaker- but when I started studying I was very rusty on the basics of grammar. I was hoping to get some tips or study advice from anyone that has used the MGMAT SC- and how they went about remembering and understanding the rules..vs simply just memorizing? thank you!