I think the "what ifs" tend to occur naturally during a crisis situation. If a person broke their leg, they may naturally start thinking thoughts such as "what if I can't work; if I can't work, how am I going to pay rent; if I can't pay rent, then......" During a panic attack, we no longer feel in control of things, so the crisis we're experiencing causes us to start thinking the worst.
Try this: Before a PA occurs, already have planned out, one or two positive affirmations to tell yourself. Actually practice saying these beforehand so they will be second nature to you if the need to actually use them arises. One of these affirmations could be; "a PA will only make me feel uncomfortable, but no additional harm will be done." You could follow that one with; "these gloom and doom feelings are only natural and will pass when the PA does."