The priority of a writer is to get stuff written and in a shape where it can be read by others. (Assumedly, submitted, sold, to agents/editors, etc.) Your goal is to get stuff written. (Whether quality or timeliness is your primary goal is something you'll have to work out for yourself.)
The advice below is fairly production-driven and deadline-oriented. However, as an artist or craftsperson, do whatever you have to do to get your writing completed. If it means setting rigid deadlines and sticking to them, then do that. If it means getting yourself inspired by writing naked in a dark room with glow-in-the-dark ink on animal skins, then go hit the craft store and turn out the lights.
Do any of these projects have a deadline? I'm guessing from the tone of your question that this isn't the case. However, for the sake of completeness: Deadlines, whether self-imposed or editorially mandated, should be your number-one priority. Respect deadlines and do your best to meet them.
If you don't have deadlines, set them. A project that has no required completion date is one that's begging to be put aside. Deadlines can seem oppressive, but they can also be inspiring (in a pressure-cooker kind of way).
Which projects are ready to be written?
Everything has the same or a similar deadline? Self-imposed deadlines not doing it for you? Let's look at project stages next.
Some of your projects are in a place where you can sit and pound out text for them, and some are more amorphous. This is great, since it means you have a clear choice of what kind of work you want to do at any given time. There's an old aphorism about what to do in the case of writer's block: When you can't get the words down, put down what you're working on and write something else.
When it's time for you to sit down and physically write out the words, I'd pick the projects that are close to being finished - you're more likely to get something done quickly. Feeling wrung-out for the day? Spend some time developing the less-complete projects - brainstorming, working on notes or outlines, or doing research. (Incidentally, browsing Reddit or Stumbleupon: These are not research.)
At the end of all of this, if you're feeling inspired to work on a barely-started project that's not due for another year, listen to your inner writer elves and work on it! With one caveat: When you're doing this to avoid a different writing project, be aware that you may have come down with Planners' Elbow, aka World-Builder's Disease: Using planning to distract from actually getting a project finished.
I's also like to reiterate: A plethora of projects in various stages of completion is not a problem, it's a benefit. When one project is giving you writer's block, you have another one to switch to.