If you have never had a panic attack you wouldn't understand. And no one saying it will be fine helps.
It just happens
I have flown many many miles in the past. My first panick attack happened as we pushed off the gate. They allowed me to get off the plane and were quite understanding. I was very embarrassed as I see these gate agents almost every other week. They said it happens more often than you would think.

I now take medication to fly - but if this is their first episode there is nothing you can do.