You want a mechanic for rash decisions? Try time pressure.
Normally players can take several minutes to decide on an action that their PC has a few (specifically six) seconds to make.
You want to emulate a condition that forces rash decisions? Limit the amount of time they have to decide on their actions. And if they can't pick an action in that time frame then their character stands around in indecision. Kind of like the "takes no action" result on the Confusion table.
Here's a simple way: Keep a half dollar around. When it becomes their turn, spin it on the table top. They have until it settles to come up with their action. And if it hits something and topples early, well that's too bad.
The time delay will be uneven, to be sure, but they'll see it happening and will have an idea of how much time they have. And when the time is up, everybody at the table will know it.
Want a faster time? Use a quarter. Want a slower time? Use something larger, like an Eisenhower dollar, or a 1 3/4 inch washer.
Lighter weight objects, like poker chips, don't spin well and lose speed too quickly to be anything like fair.
But if you want a rushed decision, rush the players.
All of that having been said, I agree with Rumbletiger: Playing a personality flaw should be a role playing thing. Give out Exp rewards or penalties for doing it well or poorly, if you want to add incentive, but limit your rules to a simple description of the behavior, without bonus or penalty involved, and let the players use these descriptions as guidelines.