This is something which helped me personally. I had difficulty saying NO and giving Negative feedback to people. This exercise really helps. All you have to do is, when you are alone, practice scolding in loud voice. Practice in a loud firm voice how you are going to say NO. if possible record it and listen to it. Does it sound like a good scolding, a good “Saying NO” . If not improve it till you are satisfied. Every day 10 minutes practice is good enough to clear the blockage of the chest and makes you confident in difficult situations.