Can you tell me how I could stop having panic attacks when I have stage-fright?

Have you considered. Cognitive Behavioral Therapy - it can be very beneficial. In the meantime- 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.