I think I've identified another factor that's made this so much more of a problem for this game than it has been in the past, which is the scarcity of actual game time compared to the time I spend sitting around thinking about it. When we're actually in the midst of a scenario, usually, I'm fine and content, and the players seem to be as well for the most part. But then the scenario finishes, and it'll be weeks -- or months, le sigh -- until we're on the next one. During that time, all I can do with my game is to sit and stew and putter with ideas.
And when I sit and stew and putter, I start thinking about all the many, many different things I'd like to do with a game. I'd like to do a Conan-esque sword-and-sorcery game. I'd like to do some swashbuckling on the high seas. I'd like to do one of the old D&D classics like "Temple of Elemental Evil." But I only actually have 10 - 30 game sessions a year, total, and so I feel enormous pressure to squeeze everything I can into that small space.
This is one of the reasons I end up dithering and changing my mind and wanting to change things constantly; it's the same reason I hate inking -- there's such a small window of "success" that I'm paralyzed by my desire not to screw it up. So I need to let go and see the game as an opportunity to have some fun, rather than worrying about Running the Perfect Game Session. A decent game that actually gets played, is far better than a brilliant game that never sees the light of day.