Low self-image usually forms at an early age. A person might have had excessively critical parents or teachers, failed to get along well with other children, or received low marks in school.
Being aware of the source of poor self-image makes it easier to challenge the assumptions upon which it is based. It is possible that while you had certain faults in the past, you presently are learning to overcome them.
Or, perhaps the people who judge you unfavorably were using a yardstick that you do not presently accept. For example, in school a student is usually judged by the marks he receives on tests. Some students with low grades worked hard to understand, and more importantly may have internalized the concepts and practiced them to a greater degree than others who received higher grades. As a child, the diligent student with poor grades might have felt inferior, but as an adult he has the ability to appreciate how he may have really accomplished more.
People with low self-esteem are frequently very good people by objective standards, but have high aspirations and hence feel frustrated in not reaching their lofty goals. Since they are not perfect, they consider themselves failures and this leads to many negative consequences.
In truth, each area of behavior and personality has numerous levels along a continuum. If you are not perfect, you need not rate yourself as a failure. Focus on improvement, instead of absolute perfection.
It is worthwhile for a person with low self-esteem to write a list of the minimum standards of a basically good person. He is then able to see more objectively whether or not he is meeting those standards.