Sorry for the late reply but as ncmako said, we've all been there. Allow me to share a few ways I've learned to overcome this common problem.
1.
Make a plan. Things like games, books, and many other creative projects have a way of growing, often too large and out of control. Without a clear plan it's too easy to loose direction and motivation.
2.
Share your plan. There are two good reasons to do this. One, if others are interested you'll be motivated to show your work and two, it's harder to give up after you told others what you were going to do. No one wants to look like a quitter.
3.
Stick to the plan. Resist the urge to change or add to the plan. Seeing your project getting bigger instead of smaller is discouraging. You can always go back when you're done and make changes, after you see the finished picture.
4.
Create clear waypoints. These can be whatever you want: stories, character placement, puzzle elements, rooms in a level or even levels themselves. Check off these waypoints on your list as you complete them. The more items on the list, the more you can check off. Realizing progress is a great motivator.
5.
Relax. Don't set unrealistic goals that set you up for disappointment. Each time you work on it, get a little done or a lot, but don't sweat it. If you turn it into a chore, you probably won't do it well.
Characters. You build them a world and what do they do? They moonwalk, get stuck in the walls and fall through the floors!
