You don't feel good about yourself. Maybe you think you are a bad person, that you never do anything right. You might not have many friends. It will be difficult for you to reach your goals. When you make a mistake, it is harder to bounce back, and makes the rough times rougher.
Mistakes are part of living.
And you have to risk failure to succeed. The biggest part of success is learning to deal with failures and understanding that they come and go. It doesn't feel good at the time, but mistakes will happen. Self-esteem helps you to be honest with yourself and with others, and still feel good about yourself. Self-esteem allows you to accept your mistakes and failures and learn from them.