Most people determine whether you are confident or not by your body language.
If you feel like you can't control your emotions. Or that you are naturally insecure and your body just acts the way it wants to. It doesn't have to be that way. You can control your behaviors and body language in order to take control over your emotional states.
Knowing the key differences between insecure and confident body language lets you choose between the two depending on which is most appropriate.
Practice shifting between the two states so that you get used to it. Next time you are feeling insecure but want to seem confident, just make the appropriate behavioral shifts and you will notice the change. Not only in the way you feel, but also in the way others feel about you.