I have a huge problem with this. I always imagine the worst happening, though the scenarios my brain comes up with are usually pretty outlandish.
When I was a JW, one of the reasons I would never comment is because I was terrified that I would grab the mike and yell obscenities into it. I also feared that I would topple headfirst over the chairs in front of me during the song/prayer and my legs would be flailing in the air with my skirt around my waist. Oof... Okay, those are the innocuous thoughts.
The ones that I have now usually center around something terrible happening to my daughter. There have been nights when I'm driving home from work and realize that we haven't talked to each other that day and I know - I just KNOW - that she's gone, that someone broke in, killed the dogs and took her. By the time I pull my car into the garage, I'll be hysterical and making bargains with God. Of course, as soon as I turn off the car, I hear the dogs on the other side of the door and then I know - just KNOW - that she's fine. The whole time I'm doing this to myself, I know - just KNOW - that I'm being ridiculous. And that when I walk through the door with puffy eyes, she's going to make fun of me and deservedly so.
It's only lately that I've been able to put a stop to it. I've found that when I start to have what I call a "Carolyn Burnham" moment (I think that's the name of Annette Bening's character from "American Beauty", can't remember for sure), I give myself a stern, silent talking-to. If it doesn't work, a sharp slap across the face and a "Snap out of it!" does the trick every time. For real.
And now for something a little lighter - my latest catastrophic thought (this one just started last week) was that I'll pitch head over heels while walking down the stairs at work and land on my front teeth. So I've been holding onto the railing extra tight these days.