I think everyone runs into that every now and then, and I think that the best solution (for me at least) is to split the project into smaller tasks:

I am very exited about it at first: this is about the time where you can lay out the plan of the project at hand, see how you will be developing it and trim it down to very small tasks (i.e. have a user/group application much like the monastery trimmed to it's basic components: nodes/users/groups/privileges . and start by developing an engine that does only that.)

when you're tired, I will safely assume that you would have laid out a basic framework. then, each time you get motivated to work again, just work on the next "small task" in the queue and focus only at the task at hand, not worrying about the whole lotta work ahead; (i.e. group->Editors can edit nodes.)

my examples are stupid I know, but I think this can generalize to just about anything. The only difficult part is seeing ahead the structure that will keep things tied together as you develop more and more on the project.