If you want to grasp English and understand all idiomatic expressions as well as its grammar and internal logic you’ll need to immerse yourself into its literature. The more you read the more you’ll comprehend. Don’t limit yourself to books that you feel are on “your level” but go beyond this and try to get a hold of more advanced reading material every now and then. This enables you to advance and learn new ways to express yourself and form sentences. Unlike many other subjects, language studies take time to settle in. Don’t try to rush it, learn it at your own speed.