If you're actually breathing the right way you can't panic it's basically not possible because it switches on your parasympathetic nervous system instead of the sympathetic nervous system. I learned that in cbt....there was this whole routine about how long you should hold it etc but seriously as long as you focus on your stomach expanding instead of your lungs it will calm you down a bit. The routine was actually supposed to train you to breath right all the time and then you would never get nervous but I found it complicated so just try breathing with your stomach. Plus I highly recommend any sort of gentle yoga cd my favorite is stress relief yoga with Suzanne deason....it's set by a beach so calming and gentle.