When you have Anxiety, why do Panic Attacks seem to worsen every time they occur?

I'm going to a class later this month to help control it, but it seems to worsen every time it occurs and lasts longer than the last one. I know I'm healthy, I'm 15, physically fit, and no Cardiac diseases from my last check-up which was no longer than 1 1/2 weeks ago. But in the meantime, what is the best way to control it? (and yes I know that Anxiety/Panic attacks have similar symptoms to a major cardiac disease) It feels like I was having a Heart attack, but as I said, i have no Cardiac problems.