This is a miserable state of mind where everything feels hopeless, we feel useless and unworthy, and if something does go well we worry that it can't continue, or we don't deserve it anyway. We have no confidence in ourselves.
The roots of these feelings are in early life, and began as shame for not being "good enough", and a longing to be different so we could really belong. We were often criticised by a loved carer, or compared badly with other people. Unfortunately this happened when we were too young to know that it was unfair, and even wrong, so we believed what we were hearing about ourselves.
The way to restore this is to:

1. Understand how it happened, and realise the injustice of it.
2. Begin to believe that you can feel differently, and imagine what you would be like if you were self confident.
3. Watch your thoughts, and notice how your own thoughts remind you how worthless you believe you are.
4. Wear a rubber band on your wrist and snap it each time you catch yourself with those thoughts.
5. Ask for help from a therapist used to dealing with this problem. Cognitive Therapy is often a good way forward.