It's just that I have a stupid way of working. As soon as the part isn't "interesting" anymore, because I have solved the main issue, I just... stop.
That's absolutely my way, sometimes, I wasn't even able to finish a work for university, means, write it down, once I understood the given subject! What a mess!
A good way to get motivated is when you have some people who depend on you finishing the thing. For instance I extended my GameEditor for a friend who uses it for a simple question-answer-game that is presented at sort of games convention. http://www.stadtbibliothek-bielefeld.de/5007.htm