Well, I always worked on multiple stories at a time but I found out that they started to bleed into each other. I see friends of mine starting multiple projects and then complaining that with those 20 projects they start yet another and basically never finish things.

I made my co-conspirator and occasional partner in crime finish some of her stories before starting on something new.

And sure, it may 'suck' if you can't write for a week because you're working on one project at the time, but even if you write 100 words a day on the project, eventually it will be alright. You can use the 'downtime' to fantasize what's going to happen next and then your muse will spark right back in action :)

Again, you can do this :)