well the obvious course of action has 2 possible outcomes - if you get someone to startle you as often as possible, it will either
a) gradually become less startling, or
b) make it even worse
perhaps worth an experiment?
otherwise maybe go with martial arts and meditation for now ... i'm pretty sure physical activity tends to distance you from purely "mental" issues - it kind of gives you a reference level for your brain to function on
i think everyone experiences it to some degree .. it seems to come and go with me ... sometimes nothing will startle me, sometimes i startle easily