Positive self-talk reinforces confidence. If you make a mistake, don't apologize. Use what you have learned to do better next time. Seek out praise. Sometimes our jobs don't make it easy to build self-confidence.
Some supervisors are quicker to criticize than to praise.
If that's the case with you, point out your successes to a co-worker who will give you the positive reinforcement you need. Build confidence away from the job. Develop interests and abilities outside of work. The more confidence you have in your other skills, the easier it will be to build confidence in your work skills. Work on your personal happiness.