Hi Haryem.
Your experience is certainly not unique. Many people find that their verbal preparation isn't working very well and wonder what they have to do differently to increase their verbal scores. Part of the issue is simply that a preparation approach that will work for GMAT quant, particularly for a person who is already strong in quant, won't necessarily work for verbal.
So, to improve your SC performance, and to score higher overall on GMAT verbal, you have to get what GMAT verbal is really about and prepare effectively.
As you've discovered, preparation that involves mostly answering SC questions and going over your errors isn't going to get you very far. In fact, what you have to add to doing mixed practice questions as you have been is do a deep dive into the concepts, working on areas of SC, such as subject-verb agreement, sentence structure, modifiers, etc., one at at time to make sure you fully understand them and then doing practice questions, untimed at first, until you are getting almost all of them correct.
For some more tips on how to prepare effectively for SC, see this post.
GMAT Sentence Correction: 8 Essential TipsFor an in-depth discussion of how to effectively prepare for GMAT verbal see this post.
How to Score High on GMAT Verbal