Part of growing up is about figuring out who we are and the kind of people we hope to become. Loads of things have an indlience on our identity, such as where we live, our family and friends, and our interests, but our identity is also tied in with our self-esteem and how we feel about ourselves. Our feelings can be positive ('I am a good friend') or negative ('I'm not good enough').
Our identity can be very 'me' focused, so our feelings about ourselves shift around as we look to ourselves or other people to figure out who we are. But God wants us to focus on him, and to know that we have a secure identity in his love for us. The Bible has loads of verses about how we are loved, accepted and forgiven, and about God's plan for our lives.