1. Your grammar is not very good (although it doesn't seem to be very bad either), so you struggle to quickly put your words into an understandable sentence.
2. Your vocabulary is not very good. You recognize the meanings of words other people use, but when you speak you can't quickly think of the words you need to use to express your meaning.
What do you think?
Don't think that you have to speak perfect English, so don't be afraid to make mistakes! Speak as you are able and you will gradually improve, especially if people like your boss will be patient with you and sometimes point out your mistakes and correct them.
Practice you English here, and ask questions. Pay atention to the grammar and vocabulary of those you think have good English. Good luck!