Why do you always have more than one writing project on the go?
This is a good question and something that's come up time and again. At the moment, I'm currently writing two non-fiction books as well as a series of short stories. And yes, it can get mind-boggling which is why there are spreadsheets and calendars to help me keep track of everything.
The reason I do it is because it actually helps me to focus. For me, switching between several projects helps to keep them all fresh in my mind as well as helping me to keep an interest in them. I'm one of those people who can get half way through a project and suddenly lose enthusiasm. Before I know it, I have a hard drive full of half written books and stories (this is also the reason that you haven't seen Book 4 from the Veetu Industries series. It will arrive at some point, I promise!).
Having a number of projects on the go also allows me to rest when one of them becomes too tough. Sometimes it's like hitting a brick wall and you find yourself living in a haze. You can't find the right words or phrases and everything feels hopeless. Having a second or third project gives me the chances to come back to the first one with fresh eyes and a rested brain. Working on non-fiction projects has reawakened fiction muses that have lain dormant for two years. On the flip side, losing myself in a fantasy world for a few hours allows my brain to think up avenues in which to take the non-fiction books.
It takes some work to juggle several projects at once and, yes, it's tiring. But it's fulfilling and, most importantly, it makes me happy.
If you want to work on several things at once, take it slowly. Map everything out using whatever you need (I personally use calendars, spreadsheets, and flow charts). Learn to step away from it all from time to time and just take time out for yourself.
"Never give up on a dream because of the time it will take to accomplish it. The time will pass anyway." - Earl Nightingale