Yeah, the mental part does not get discussed very often. I know that I was a little concerned about this aspect of the tour when I did my solo TransAm last year. I'd read stories of other riders losing heart and bailing on the tour. One woman whose journals are on Crazyguyonabike quit on her first try and regretted it and so went back a year or so later with a greater sense of resolve and finished. We enjoy aspects of the tour, but finishing some of the really big projects is part of the challenge and reward, too. And when the going gets tough--as it most certainly will--it's the mental part of the game the counts most.
As another poster mentioned, I never wanted to quit, but holy cow were there some low times, especially in western Kansas battling horrendous side winds, farm trucks, and a horizon forever out of reach. Getting through those days was all about commitment. A really neat kid was inspired by my journey when I met him in central Ohio. He insisted that he have his picture taken with me and the bike. Later that evening, he ran by my camp and dropped off some water and yelled, "Never give up!" as I lay sweating in my little tent. That was my mantra whenever I faced some daunting days in the saddle.
Long tours are life in a distilled form. To succeed, we need the same skills as in other spheres of activity--rolling with the obstacles, knowing that however unpleasant the current moment is, good times will come again.
My 2 cents.