If You Don't Know, Say So
Many people will perform mental gymnastics to avoid saying, "I don't know." Depending on the context of a situation, these three words can be very difficult to say.
Every time you don't know something that you're asked about, have the courage to say, "I don't know." View every time you repeat this three-word sentence as another time that you are increasing your level of courage.
This way, instead of feeling embarrassed that you don't know, you'll be able to focus on the fact that you are increasing your level of courage.