No other phrase – no other way of thinking, actually – is more pernicious for people with anxiety.
“What if?” is the corrosive fluid that invades even the tiniest cracks in your resolution. It carries the implicit worry that sustains so much anxiety: something will happen and you won’t be able to handle it. “I know it wasn’t a heart attack last time but what if it is this time?” “I know the neurologist said nothing was wrong but what if something’s happened since then?” “What if I faint in public?” “What if they laugh at me?”
It’s easy to convince yourself that you’re identifying real problems when you ask this question, but you almost never are. The “what if” itself is the problem. Whatever good it might do to ask “what if” is far outweighed by the mental anguish maintained by its prodding. Best not to indulge it.