Hey no you are not alone !! The loss of motivation seems proportional to the amount of time spent on the project (at least for me) : my latest game, Froogz, took me nearly one year to complete, and I really don't want to see it anymore for now !!!!

Also motivation can be boosted by some users or friends who get involved in the game (testing, design).

About writing a design doc, in my case it won't have helped since I hate writing docs, so this would have added more to the loss of motivation for the project (another boring task ...).

Another idea could be : work on several small projects at the same time, then switch to another one when the current becomes boring ?