Often we are undermining our own abilities by our own pessimistic attitudes.
For example, a colleague of mine was supposed to be sent for a computer course to learn autocad, but instead of grabbing this opportunity with open hands, she shot herself dead by saying she is too dumb to learn such high end stuff. Then she complains that she works so hard why the company does not appreciate her work.
Which is what Marie Callas advises us not to underestimate our own abilities.
She says each time we tell ourselves we are not up to it and put something off, we strengthen our own beliefs that we are not capable of tackling these tasks. Tasks can be as simple as cleaning your house, passing an exam, learning a new language to complex ones such as managing a mammoth engineering project.
So whenever I notice I'm starting to think of a reason not to do something, I just force myself to start doing it, right away. For example, i dislike to do housework but the urge to complete the task ahead forces me to move my body towards the closet, open the door, grab the vacuum, plug it in, and start the cleaning process.
In the end of the task, my body feels lifted, although I am perspiring.
The right time to do something is when you notice you are trying to avoid doing it.