The biggest confidence killer is CRITICISM, in all its forms including self-criticism and “constructive criticism.”
Do you have a person in your life who uses a criticizing, shaming method to “motivate” you to lose weight, get a different/better job . . . ? How does it make you feel? Confident and self-trusting or defensive and self-doubting?
If you take their criticism to heart, you will actually start to believe it which WILL eventually kill your confidence. Be careful about what you allow. I hate the term “constructive criticism.” To CONSTRUCT means “to build or to build up”; and To CRITICIZE means to point out faults, flaws and to tear down. You cannot build something up while tearing it town. It is not possible.
Teach critics how to help you in a way that is not emotionally harmful. Give them specific words and phrases they can and cannot use. This works with family, friends, co-workers, even employers. If they will not comply, inform them that they do not have your permission to criticize you. It may shock, surprise and confuse them but it will stop them.
Check yourself when it comes to being critical of others. We often attract what we are.