Here are three useful strategies for you that should help:
1) Focus on learning the most commonly-tested points of GMAT grammar, which are Subject-Verb Agreement, Pronouns, Modifiers, Idioms, Parallel Construction, and Verb Tenses. This will narrow your focus and help you get more right answers, since these are the rules that you can expect to see most often. Don't get lost in the weeds of arcane English grammar rules that make textbook writers sound smart but that appear in less than 1% of the sentence corrections you're even likely to see;
2) Learn strategies like the Bracketing Technique (see: https://youtu.be/QdMs09uwP2g ) and other methodologies for weeding out the confusing aspects of a sentence, zeroing in on exactly what you should be evaluating from a grammar standpoint, and eliminating clearly wrong answer choices. Correct methodology is as much a part of effectively tackling GMAT Sentence Corrections as are the grammar rules themselves;
3) Pattern recognition is huge! You need to teach yourself how to look at a sentence and QUICKLY recognize what major points of grammar are being testing. For example, on the first read-through of a sentence, you should have a pretty good sense of whether pronouns are at issue, or misplaced modifiers, or subject-verb agreement, etc. This is a skill that can be learned and practiced: https://youtu.be/NH0ZByEeDY0.
I hope this helps!