I recently read about the "hedonic treadmill" which essentially says that people revert back to a basic level of happiness despite whatever is happening in their lives. This is something which I kind of intuitively realised since I was a kid (I used to call it the "zero law"- in the end the positives and negatives tend to balance out).
I wonder if this could kind of explain motivation issues. You get an idea for a project, something which looks great and exciting and are highly motivated to start activity on it. Then, because of this hedonic treadmill thing, you get used to the idea of the project and thinking about it doesn't give you as much pleasure. This leads to less motivation for the completion of the project. Add to this the many inevitable setbacks which will be experienced (please don't forget though the serendipitous discoveries and new ideas on the way too- things you wouldn't have realised unless you actually did work on the project).
The solution I propose is that we just accept this "hedonic treadmill". Accept the fact that the stuff on our NA lists will sometimes repel us, bore us and make us downright unhappy! But remember that (assuming you're doing your weekly reviews right) all items on that list are the best things you can possibly be doing with your time. Forget about rewarding yourself for completing an unpleasant task. The hedonic treadmill works the opposite way too. The more unpleasant the unpleasant tasks are, the more pleasant the pleasant ones are! And you don't need to motivate yourself to do the pleasant ones!