How can I improve my writing?

I am an Asian American. I have been here 3 years ago. English is not my first language. I am currently a freshman in the high school. My English teacher says that my writing is awesome despite some of the grammar errors. I admit that I am a good writer, but I also know that my grammar is horrible. I really need your guys' help. Does anyone who has any idea of how to improve my English grammar. By the way, my grammar problem is not like verb agreement or tense, etc, it's more like the sentence structure (awkward sentence order, syntax, etc.). Here is my example: (my thesis for my English essay) The author illustrates the theme of the novel by describing two main characters: A and B, and the plot of the story. But then, my teacher changes it to: The author illustrates the theme of the novel through the plot of the story and by describing the two main characters: A and B. The problem that bothers me the most is: when I check my essay, it always looks perfect (I mention the grammar here), but my teacher can always find out tons of grammar issues. Thank you so much if you help me to improve my grammar. Also please check the description I just wrote if you would like to.