If you are alone or in your comfort zone, there is no worry. Because you feel yourself safe. If you feel that you have to show a speech performance, such as answering a question or giving an important information, your subsconscious tells you that you can't express yourself efficiently. After that signal, other danger signals come: People will judge and humiliate you. If you free these feelings, you can remain calm and reduce blocks at the beginning of sentence, especially if you use a good airflow.