You have to affirm and feel it continuously. Then physically act like it, through all mistakes until it becomes a habit. Do not feel discouraged if a lack of confidence appears at some point. You must be able to forgive yourself and keep affirming the new habit. Forget about the embarrassment, and persevere. The old habit of lack of confidence, or past failures is irrelevant. Do not let it hold you in bondage because everything you get in life requires self confidence. Also, train yourself to be immune to the behavior and negative attitudes of others. They have no power. Don't let their negative behavior destroy your confidence. Just ignore it. And keep affirming strength and self confidence. Remain at a higher level of thoughts and feelings than the society. You can also use visualization or self hypnosis for faster results.