My professor marked "word choice" on 20 places in my coursework. How do I build better vocabulary?

Hello. I am international student from South Korea. I study international relations. I have been in USA for 2 years. My English is better now, but my writing is still problem.

I got coursework back yesterday. Professor wrote good comments about my ideas and arguments. She said I understand the concepts. But she circled many words and wrote "word choice" or "this word is not quite right." She said my writing is "slightly off" in many places.

This is so frustrating because I know what I want to say. In my head, the sentence makes sense. But when I write, I use word that is close but not exactly correct. Like last week I wrote "The government demanded the situation" but I meant "The government addressed the situation." Two different meanings!

I use dictionary. I use thesaurus. But thesaurus gives me words I don't know how to use naturally. It says "angry" can be "ire" but when do I use "ire"? Never? In poetry? I don't know.

My question: How do you build vocabulary for academic writing specifically? Not just memorizing words, but understanding when to use them and how they feel in a sentence. Do you read academic journals and copy sentences? Do you make flashcards? I need practical strategy, not just "read more." I read a lot but I still make mistakes.

Also, how do you learn collocations? Like why we say "strong coffee" but "powerful computer"? This language is crazy sometimes. Any help for struggling ESL writer? 감사합니다!

I'm a linguistics student and collocations are literally my favorite thing. "Strong coffee" vs "powerful computer" is about history and usage patterns, not logic. It's frustrating but learnable. What helped me as a language learner: when you learn a new word, learn it WITH its common partners. Don't learn "address" alone. Learn "address the issue," "address the situation," "address concerns." The word lives in those phrases. Learn the phrases, not just the word.