Have you ever wondered why dieting doesn't work? Or why most people let their gym memberships lapse shortly after joining? Or why that positive thinking book or seminar wears off a few weeks after you attended it. The reason is usually that you haven't really committed to change. You have decided to "give it a go" or you have set yourself a short term goal.
One of the biggest errors people make is trying to bite off more than they can chew. This often means that what they are trying to change is often simply too hard or there isn't enough margin for error. Or even worse they don't really believe that they can make the change stick.