Focus

Why is it that I'm incapable of focusing on one project at a time? I have the book I'm working on -- that is going fairly well, mind you, save for the characters still being sorta flat (but I figure I can fix that on revision, once I know them better) -- but I've got a great idea for another book, that I've written half a synopsis for. (I'm sure the synopsis will change as I write but I figure this might give me some guidance for when I actually start writing.)

It's like I get bored with what I'm working on and want to move on to something else, but that isn't very conducive to actually completing a book and trying to get it published now is it?