How can i stop a panic attack while it's happening?

Relaxation breathing: You can teach yourself relaxation breathing by reading and applying the book: the relaxation response by herbert benson. Otherwise, the best thing to do during a panic attack is nothing. Let it go away on its' own and it will eventually go away. This is called desensitization.

Agree w Dr. Passer: If you can find a quiet place - do so. Breath in slowly & deeply through your nostrils. Fill your lungs with air (but not to the point of discomfort). Hold for a count of five then slowly begin to exhale through an open mouth. Repeat at least 12 times, longer if you need to. If you haven't been assessed for your panic/ anxiety - that would be prudent. Anxiety is no fun BUT it is treatable.