Personally, when I am working on a project, I find it is easy to lose my way when I reach the point of knowing I am capable of finishing it -- even if still in planning. It interests me more to know that I could do it than actually proving it to others by completing the project.

I find it useful to examine what is left of the project and, GTD style, break it down into next action steps. With the remaining tasks broken down as small as possible, I find it easier to stay focused and knock things out. As others have mentioned, the motivation at this point is often social; I don't want to seem uncapable to others.