It doesn't matter how long I have learned English, I still have less confidence in using English. This issue is not about my personality and and trait. One of the reason is that there is no scale for the confidence of language ability. In my case, when I talk with my English teachers, some native speakers in an English training center, we can understand each other. But when I watch the serial FRIENDS, I cannot understand much of the conversations. I told one of my teacher that I had been reading for novels or other books by native writers for a long time. But he watered me down, he told me that it was impossible to understand everything in English and I should learn real English. I could understand what he wanted to express. It is true for some native speakers that they neither haven read much in English nor have learned many new vocabulary words, but they are confident with their native language without doubts. That's the confidence that I am looking for.
So my question is, how do you define your own confidence in English. On other words, how did you get your confidence in learning English?
I am not a native english speaker but I am fluent in it. I guess language is something that's born within. It doesn't mean that you should stop reading novels or watching drama series because in order to really get a grasp of the language itself, blending in is always the best way, that is, to speak, to listen and to read.
From what you have written, you are pretty articulated when expressing your ideas, though it also appeared that you have some grammatical errors (ie. one of the reason(s); misuse of prepositions).
One thing for sure is you don't have to compare yourself to the native speakers because it's their mother tongue. It's okay if you are not 100% comfortable with english / not 100% confident about your english, because guess what, others learning mandarin or some other foreign languages might feel the same way too!
Just speak up always. No one's going to laugh at you for occasional grammatical mistakes and such! The most important thing is to be comfortable with the language and always enjoy it from within.
Hope that helps!