Affirmations
The mind is programmed by what is put into it. Mostly this is from the continual chatter that goes on in our heads. Very often this is negative -"I'm not good enough.……etc." Affirmations are a way of deciding what we want to become or a way of setting a goal we want o achieve.
Start with an achievable goal. If you make the objective too unrealistic -"I want to be a millionaire tomorrow" -then you will probably build in disbelief and disappointment too.
Lack of confidence is a common experience for many people so perhaps start with "my confidence increases daily". Use it as a sentence in your head as often as you can. You may want to apply it to a specific area of your life such as work -"my confidence at work increases daily." We are what we think. Think what you want to be.