I Want To Be Confident

Am a person with a lot of potential in many things but i lack the courage and confidence. In several occasions, i have been appointed to head various departments but i always end up not delivering as expected of me by the directors. What make me really sad is that other people always sees the potential in me but i lack self confidence . I cant even explain something i understand so well clearly without tensing and thinking that i will end up making a mistake. I have this unknown fear which is a major setback in my life. I want to believe in myself and to bring out the best me.