Here is a list of four very good possibilities keeping you there:
1. Under earning gives feelings of moral superiority
2. Under earning is caused by low self-esteem / low-confidence in who you are and your value
3. Under earners believe unconsciously someone is going to resume them
4. Under earners believe there's virtue in not having money
Exercise to begin the change:
List every way you are under earning in your life.
(No matter how hair brain what you write down, what appears is there. Because it can not appear if it isn't there.)