You are the product of all your experiences in life, good and bad. All these experiences combine to form belief systems within you, which dictate how you think, feel and behave.
Inevitably, in amongst the positive beliefs you have about yourself, you will also have some irrational, negative ones.
Negative beliefs not only hold you back, they also cause you lots of stress. You can pick them up at any time in your life and until they are repaired, they will continue to cause you problems.
To repair negative beliefs
Negative beliefs are repaired by bombarding your subconscious with images and ideas that are the opposite of what you fear. If, for example, you lack confidence, imagine yourself as supremely confident and your subconscious will believe your imaginary experience to be real.
Doing this once will have only a small effect... Do it every day for at least 14 days and you will notice a change in yourself... Do it every day for a month and you will have permanently changed what you believe about yourself... and you will be a different person.
Your subconscious, which controls all of your automatic behaviour, cannot distinguish between what is real and imagined.
So bombard it with images of 'happy you', 'healthy you', 'successful you', 'confident you',... and it will believe them to be real. This will change what you believe about yourself and what you believe, determines how you think, feel and behave.