You’re right. Sometimes I get this trouble, too. Each time, when my teacher tells me to talk about something in English, I always miss my words. I mean in my mind, I can think about that word but I can’t express it in words. It’s quite hard for you to understand what I say, right? Anyway, my teacher told me that if I wanted to improve my speaking, I had to write down anything I wanted. And it must be done everyday. Maybe he’s right. So if you think it’s good, let’s try !!! OK? See you later!
I agree with Thegladiator. This is not a unique problem to the English language, but rather to second language learners everywhere. As you begin to learn the language, you tend to understand more than you can produce (that is to say, you receive lots of “comprehensible input” but are limited in “output.”) Some instructors believe you should begin production right away, yet there are many language theorists who insist there is a “silent phase” of little output while you are processing a lot of “input.”
I think you have to be patient with yourself and expect to make many mistakes when you first learn English. Go ahead: speak and write, but don’t get upset if you aren’t 100% correct. It’s best to make a mistake and learn the reason why it is incorrect than to produce perfect English and not feel confident in your understanding. Mimic the patterns and structures you hear, and test them out on a native speaker.