Assertiveness training is a good way to learn to stretch your comfort zone" - meaning to take small risks, tiny steps in facing your fears. Every step taken enhances your confidence and builds your strength and ability to challenge your fears. You may start with phoning a business in town to register a minor complaint. The next day you progress to talking with a friend with whom you've had a disagreement.
It takes courage to confront fears, so it helps to share them with others and give yourself lots of positive acknowledgment for each bit of progress realized. Keep in mind, however, that the point is not to eliminate fear, which is a natural human response. The point is to recognize it and befriend it. People who accomplish great things are those who move forward with their fears, not because they have no fear.