I see that you can express yourself in English, at least to a degree. So, congrats on putting your situation in words.
By the way, it´s not your fault for having such difficulties, the teaching in our country in general doesn´t deal with those issues, not in a proper way. We don´t value the teaching of Portuguese, to begin with, let alone English, Spanish and other (at least the Romance - Latin- languages).
On the other hand, we have plenty of people around that are in the know. I know for a fact that we have native, advanced, and expert members. Not to mention the ones that use or had contact with English in English-speaking countries.
Don´t sweat it. You are not alone, you are in good company, many of us have issues with punctuation, tomorrow or later I will suggest a book with that topic, anyway, count on us to polish your texts (at least a little bit.)
As for anxiety, there are lots of posts with information about conversation and learning in general, run a search on the EnglishExperts itself and you will hit paydirt (vai achar muita coisa boa).
So, don´t get anxious, the other people is a human being just like you, and with the same curiosity about you as you have about others.
On the topic of vocab, it comes with time, training and a drive for variety (for expressing things in more than a way, or in several ways.), but don´t get in a hurry, practice makes perfect!
Don´t compare you with others, whether more advanced or less, compare with yourself. Compare your improvement, how you stand now, and where you got, two-three-or six months from now! You´ll see that you worked hard, learned something...and that will be rewarding for you, believe it or not!
See you around.