What is the most stressful part of being a CESL student? Why?

I have the most stress when I can't tell something that I want to tell, because I can't imagine a word in English. Actually I am an outgoing person in Korea. But in the USA, I'm a shy guy, because I'm not good at vocabulary. Even though I have some questions in class, I can't ask them to the teacher. This situation makes me silent. And my poor vocabulary ability makes me not able to hear. That's why I have the most stress in CESL in the USA.