How to tell my supervisor that I have severe phobia of flying?

The phobia of flying is discouraging me to travel for conferences, although quite a number of them are very good opportunities for me to "market" the researches of our lab and of mine. Apparently, I can't overcome this phobia even with the help of therapists. I almost passed out due to panic attacks on the airplanes, and after the flights, I always felt too exhausted to do anything for an entire week.

I think it's time to let my supervisor know that I can't to go to any conferences that I have to fly there. How can I do it in the most professional way?

Thanks