Confidence is something that can be lost and gained through life experiences but can also be changed by reinterpreting life experiences. For example, if we get made redundant, it could really knock our confidence. However, we could reinterpret it as it was the Universe giving us a helping hand to get out of a job/career that we weren't suited to so that we could find a job/career that we really love. And being dumped by a partner, instead of making us feel less confident about ourselves, could be reinterpreted as an opportunity to heal ourselves so that we can then attract a partner who really gets how great we are.
One of my clients was in a relationship with a man who was constantly finding fault with her, who was sucking her dry financially and they kept splitting up and getting back together. To start with, I had to give her a reality check with regard to relationships and what a good relationship is actually like so that she could see how different her relationship was. Then I worked with her using hypnotherapeutic techniques to build up her self-esteem and confidence. She then reconnected with another man whom she was quite attracted to but he had previously been in a relationship. This new man let her know that he was interested in her and that he was now single. She was then able to break the pattern of going back to the original man. So, when her original boyfriend rang her wanting to meet, she was able to be assertive and say no.
There is also a phrase "rejection is protection" so if we interpret someone rejecting us for our benefit even if we don't understand why at the time, we can feel much better about ourselves. For example, in my own life, I was in a group of healers and got rejected by the group and initially felt very shocked. But in hindsight, I can see that belonging to the group was keeping me small as I was not valued and not able to give all my gifts.
Hypnotherapeutic techniques are not the only way to build confidence but they are certainly a very effective and permanent way.
I also recommend doing personal development or for those people who cannot afford therapy, I recommend subliminal software for the computer.
I use this software constantly to support me in growing and achieving more plus clients have reported that they feel better, get more done, and have improving relationships when they use it on a regular basis.