A belief is something that we think is true. Usually taught to us at an early age or something we learn through religion and media.
Once a belief is established it is difficult to change. Not impossible but it takes time.
Unfortunately if you want to manifest immediately that old belief is working against you.
This process involves pretending as if your belief has already changed.
Pretend that you believe in what you desire. For example you may want to find a new lover but have low self esteem with the belief no one can possibly love you.
Instead of using that belief of unworthiness imagine what it would be like if you were worthy and did love yourself.
How would someone who loves themselves behave? How would they walk and talk.
Make a list of the types of behaviour a person with self love and confidence would have. Everyday practice one of the things on your list. Maybe you will look people in the eye when you speak or go out after work for a drink.
It maybe difficult at first but with time and practice you gain momentum. Law of attraction does the rest.