Recently, I have* been taking a TOFEL course, which makes me feel like (I’m = you don’t need, because we already know you are the subject “I have been…”) idiot.

One of the most difficult thing to do (Although we know something is difficult and the object is ‘memorizing words’ you need the ‘do’ verb, in order to make sense) is to memorize words.

It was a surprise for me that there are so many words in (instead of ‘of’ because = words are ‘in’ the field of study’) each study field that that I didn’t know before (=that I've not known before, OR “Words that I haven’t come across before”)

I even knew what 'hydrothermal solution', 'exotoxin' means, as a Korean first.

If I forget my language, would I be good at using English...?

It's a long way to get a good grade.

* “Recently, I'm taking a TOFEL course” = If you omit “Recently” then the sentence makes sense. “Recently” indicates that you did something in the past. Usually in English we will add another sentence to indicate present progression “Recently, I started taking a TOFEL course”. = “Lately, I have been sick” (I may/ may not still be sick).

“as a Korean first” = I didn’t understand what you meant; Do you mean “I didn’t know what Hydrothermal Solution or Exotoxin meant in Korean”?? I understand Korean so maybe if you write it in Korean I can understand better.

Also, I’m sorry you feel like an idiot. Learning a new language is difficult but don’t give up. Everyone struggles to learn a language at some point. You are not only learning a language, but its culture and learning about its society. Just because you cannot remember words, don’t give in. You don’t have to be perfect! No one is. I always struggle to remember Korean words, there are so many. I can explain things in English clearly, but in Korean I sound like I child. If you give up now, you will never know what is possible. 
또는 당신이 바보처럼 느끼다고 말하는데 불쌍해요. 새로운 언어를 배우는 것이 어렵지만 기죽지 말아요. 모둔 사람들이 어느 시기에 새 언어를 배우기 투쟁하잖아요. 당신은 언어를 배울뿐만 아니라 그 문화하고 사회 관련 배우고 있어요. 단지 언어를 기억하지 않아서 기죽지 말아요. 당신이 하는 모든 일에 완벽해야 할 필요는 없어. 당신도 알다시피, 누구도 완벽할 수는 없어요. 저는 항상 한국 단어를 기억하는 것을 힘들어요. 너무 많아요! 제가 영어로 것을 설명하 수 있지만 한국어로 말할 때 아이처럼 들려요. 지금 당신은 포기하면 뭐 가능하는지 절대 알지 않아요. 계속 화이팅!!

If you have any questions feel free to ask!