I have NEVER met someone with the exact same problem I had! I was constantly afraid the I was going to black out or pass out and I never even knew why it was so scary. I was in 8th grade when I developed the phobia. My fear or passing out due to a panic attack was so bad that I couldn't go to school anymore. I thought that heat, excersize, too little food, most anything would cause me to faint and I avoided any of those situations. My problem got so bad that I even developed depression and wound up in out-patient treatment. I had a counselor there that told me something VERY valuable. She said that the one thing that your body fears the most isn't going to happen to you. Because I thought passing out was the worst thing that could happen, my body wouldn't allow it. I slowly was able to go back to school and had a chair placed outside the door so I knew I could leave if I had to. Slowly working your way back into normal situations is the best way to go. I really do wish you the best of luck, and I hope you find the right solution for you. Pleaseee contact me if you need anything, I'm super glad I found another person just like me.
Sincerely, Annahleise