If you have promised to do something for someone but do not feel you are capable of doing it you lower your self esteem. You then fail to do it and let not only the other person down, but also yourself. 

This becomes a vicious circle because in not doing the job you have confirmed your own belief that you cannot do it. By not doing the job you feel a failure and are less likely to take on other jobs and in doing so you feel a failure. When people see that you cannot be relied on they are less likely to ask you to do jobs.

Despite not believing in your own capabilities you resent the fact that you are no longer asked to do tasks. The internal chatter starts in your mind that you are worthless and the rot has set in.

The reality is more likely that you could do the job well. You do not have to be perfect. Too often people will only try a task if they can do it perfectly in record time. A task can always be tweaked once it is completed.

 Break the major tasks down into smaller ones that are not so onerous. Do not look at the required task as one major one. Instead see it as a lot of little tasks that will join together to form a bigger one. As each step is completed it will increase your self esteem.

As you gain confidence in yourself you will be more willing to take on more responsibility. More people will have confidence in you and you will be ask, and be able to do, more difficult tasks which you will be able to complete successfully.

You will be more proficient at breaking tasks down into smaller ones and also at devising an action plan to complete steps in an order that avoids duplicating work. You will be able to set up systems and check lists to make your work easier.

If we only do tasks that we are sure we will succeed in we will not be able to move on in any area of our lives. As we become more successful in one are of our lives our self esteem grows and we are more willing to take risks in other areas of our lives and are less likely to procrastinate over any job that has to be done.