Yeah, this is exactly the pitfall I'm trying to avoid, myself! I have a bunch of different long-form stories in waiting, but I'm pretty sure if I don't pick one and stick with it, I also won't finish it. (It helps that my medium of choice has a widely-utilized serialized format--webcomics--which makes switching projects especially hard and should keep me honest/focused. Hopefully.)

Deadlines are tricky. I try to use them to force prioritization, but that doesn't always work. /o\