Pay attention to the little voice in your head that says things to you about yourself. Listen and determine if that voice is giving you positive, constructive feedback that you can use or if it's merely negative and destructive. Sometimes as children we develop deep feelings about our worth and where we fit in, in the world. These feelings are not always positive. It is those negative voices that we need to balance.
Write down those core beliefs so that you can seriously evaluate them and determine their source. If you don't know what your core beliefs are, you will never be able to balance them. Balance is critical as we age so that we can let go of past situations and evolve as human beings. An example of a negative core belief is, I'm not good enough or I am worthless, which stems from our feelings of self-worth or lack thereof.
Identify any problems in your current life your negative core beliefs may be causing. If that little voice in your head is constantly telling you that you have no worth or that you're not good enough, you need to find a balance. Chances are you're not reaching your full potential in many areas of your life if your core belief is that you have no self-worth.
Focus on when those negative core beliefs come into your head and what triggers them. Make a list of those triggers. Maybe they come to mind when you are trying on clothes and you feel unattractive, or maybe when you apply for a new job or meet someone new. Whatever the triggers, be aware of them so you know when to be ready to counter the negative self-talk with positive.
Make a list of positive core beliefs to counter the existing list of negative ones. These beliefs are what you will use as ammunition every time a negative core belief pops into your head. You'll be in a situation that is a trigger and you'll have handy a list of positive core beliefs that you can tell yourself. Although it may be important for you to acknowledge the presence of your negative core beliefs, you also need to know how to dismiss them. This way, you are accepting what damage those negative thoughts have done in the past but you are no longer listening.
With practice, replacing old, negative core beliefs with new ones will become a habit, and before you know it, you'll find your own core beliefs shifting in a positive direction. The process doesn't happen over night, but with focus and repetition, you will find yourself thinking more positively about yourself and feel your mental health improving. The goal is to achieve balance within yourself.