A lot of us studied English from secondary school through university. We can recognize a tense. We can read a paragraph. We have a private pile of vocabulary. Then a stranger asks a simple question and the mouth does not cooperate. The embarrassment feels like a verdict on intelligence. It usually is not.
We practiced the silent skillsLink to this heading
School rewarded the skills you can do alone and on paper. Choose the correct answer. Translate the sentence. Memorize the list. Speaking was rare, public, and easy to mock, so people learned to stay quiet and still pass. Ten years of that produces exactly the person who understands the question and answers late, or not at all.
Fear of the wrong sentenceLink to this heading
The classroom treated a mistake as a mark against you. Conversation treats a mistake as the price of the next sentence. If you wait until the sentence is perfect, the turn is over. Native speakers do not speak in finished essays. They repair as they go. Students who were trained to be correct before they are audible never get the practice of repairing.
What to changeLink to this heading
Speak on purpose, in short ugly sentences, with a person or a recording that answers back. Care more about being understood than about the tense you would have chosen on a test. The grammar you already know is not wasted. It was just never asked to survive contact with another person's face. That contact is the missing course.