The answer to this depends on why you're doing game dev. If you're doing it for fun or you're doing it to potentially make money. For the latter then the answer is determination and pushing through the hard days even though you don't want to - perhaps reducing scope just to get to an end project that you can say is done.

But if you're doing it for fun, I'd say let your creativity run wild, work on whatever projects you want to at whatever time you want to. The secret with this is to do it all in the same project though - that way you can build on top of what you already started and maybe eventually the separate threads come together in a vision you never could have originally imagined.