We are usually held back by internal dialogue or by having too much to do in too little time. We have difficulty in prioritizing things so that we do what is most important at the time. Lack of good organizational skills can hurt our progress much of the time.
In other cases we are afraid of actually doing something because we risk ridicule or being judged based on our efforts and results. So part of us feels like if we do nothing we are not placing ourselves at risk.
At other times we may feel we lack the knowledge and / or experience to get something done right and may put off something until we get either more knowledge or help from outside sources.
These are the most common reasons for putting things off although there might be other factors involved as well.
David V. Peters