I know many English words but can't think how to use them

You have the process backwards. We don't choose a word the try to use to say something. Rather we think what information we wish to convey, and select words that best match. Practice is the key to everything in language. Reading more grammar books and learning more vocabulary won't increase your English ability. You need to speak and write and take in the feedback. Even if you get the grammar wrong and only use simple words, that is fine. As you practice more, you will get more fluent and more confident using the language. Then you can absorb new vocabulary as a baby would do, by hearing the word in context several times and inferring the meaning from there. That way context comes along with vocabulary, so if you have learned a word, you also know how to use the word. Learning words on their own, without context, isn't really learning the word at all.