When does a writer know if he's stretching himself too thin?

I've been hugely busy the past couple of weeks working on is the artist's references for my next Osprey Publishing book. This one is on the Gunfight at the O.K. Corral and the ensuing Wyatt Earp Vengeance Ride. As with all Osprey titles, an artist is painting three color plates just for the book. Part of the writer's job is to describe everything in minute and historically accurate detail. While it's fun to design paintings, with my level of artistic achievement it's certainly best to get someone else to actually paint them!

Although the book will come out nicely and probably be a hit with Osprey readers, this work is making my other two writing careers suffer. As regular readers of this blog know, I am a history, travel, and fiction writer. My output of travel blogging for Gadling has been less than normal. I have been working behind the scenes, however, planning what will be my greatest adventure travel series ever. More on that in a month or so. of course, that series will make my fiction and history writing suffer. SIGH.

My fiction writing has been hardest hit. I've written perhaps a thousand new words and edited a few dozen pages in the last couple of weeks. That's about a day's fiction work on my normal work schedule. I also haven't been able to do any promotion, not even properly announcing my birthday sale except in a few venues.

I feel stretched thin. Are three writing careers too much? Should I focus on just one or two? That would be ideal except (A) I can't afford to give up any paying gig and (B) I don't want to. I love all three of my careers and want to keep them all.

So what to do? There are only so many hours in a day. Do I just bite the bullet and accept the fact that while I have three careers, only one or two can progress quickly at any one time? Is anyone else in this situation?