Don't focus on what scares you about your new role or task. Don't take it as a threat, but as an opportunity to achieve something. Keep telling yourself what a great opportunity it is and that it's not so different from your previous assignments.
Goals don't have to be just big and scary. Every big project should to be divided into parts - individual steps you can achieve gradually.
If you want to further develop your managerial style, watch other leaders around you. Do not limit yourself to only one model, look for different styles.
Non-verbal cues can reveal your lack of confidence quite quickly. That is why you should learn to take confident postures. These include upright posture or seating positions, striking your chest, holding your head up, etc. You can read more on the concept of "power posing" in this article.
If you really are too busy or strongly believe that you will fail, faking self-confidence is not the right thing to do. The purpose of faking confidence is to push your boundaries and to improve, not collapse. If your new task really is unrealistic, say so and refuse to do it.
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Article source Harvard Business Review - flagship magazine of Harvard Business School