Once I have been stuck in the same situation as yours. I could speak out in English classes in my English training center, also in online English lessons. But once back me workplace, I could not speak out a word to my foreign colleagues. Until one day I stayed with a director who was from Texas, United States. in the elevator. Only two of us were in the elevator. Must see something, I thought. Then we talked about the lunch, and something else. I knew exactly what he said, but I didn't know how to response. No time for prepare. I just spoke out some uncompleted sentences. But it was okay, I thought he didn't notice the struggle in my mind. So from then on, I didn't fear to speak to native speakers. The worst situation is he or she cannot understand me. So I think I have overcame the fear to speak native speakers.