irrational fear.  I grew rideing quite a bit around here.  I did a couple of trips on horseback with the family as a kid out west too - 10+ days.  It's been about the last 10 or so years that I've picked this up.  Unusual because I whitewater canoe and kayak, have no proble pointing my noggin straight down hill on a pair of skis or on a mountain bike, etc.....  So I really think it is a control thing... or I'm not in control thing because the horse has a mind of its own that just doesn't jive with mine.  I still give it a go once or twice a year at a family members farm.  On just such a quarter horse - Black Bart, but he's a sweety I think.