Welcome Peetiedog. sorry, i'm a bit late to thank you for the question. tombombodil summed it up nicely though. I agree that the trick would be to gently remind her, and work with her to get her mind off of it. Don't force her to completely stop, or the irrationality might just get turned on you, but maybe take her hands, lead her to a nearby table, and see if she would rather help with a sodoku or something. Especially because you note that it is stress-triggered, trying to get her focus on something else ought to help. I'm not a professional, but the logic seems sound that if her mind is occupied until she calms down, that would allow her to better manage it.
"It is easier to outsmart people than to outdumb them" - Scott Meyer (as Omnipresent Man)