¡Hola Fernando!
Don't worry. It's common to see some performance drop at this time in your studies. I have two suggestions. First, move on to another aspect of your GMAT preparation and come back to sentence correction later. Give yourself some headspace to build your confidence back.
Second, think about how you best learn, and make sure that your preparation work matches your preferred learning style. What I mean is, some people are very good at absorbing information by self-directed reading. Some people are better at learning through listening, and some are better at learning through watching.
Our GMAT preparation course is video-based. The sentence correction section has about a half-dozen FREE lessons and practice questions that you can watch to see if they are helpful to you, with no pressure to buy from us or anybody else:
https://gmatprepnow.com/module/gmat-sentence-correction
¡Que le vaya bien!
