The "gap thing is a big one for me too. I've walked an extra thousand miles to avoid walking on the sidewalk grates here in DC> My mother tells me that as a toddler, I refused to walk across the bridge between our house and the park we went to daily. Even when someone carried me, I would scream in terror. I've had nightmares my whole life where I accidentally walk off of a brightly lit pier and fall into deep water. I love swimming, and plane rides and views from the windows of tall building don't scare me in th least. It is definitely something about thefear of involuntarily (?) throwing myself off the edge. Maybe clumsiness is what I truly fear! Whatever it is, it sucks and i's getting worse as I ger older. I wonder why.