You may ask why is it? Because in that concert hall I had to play all music by heart. Sometimes I still have nightmares in my dreams that I'm performing and suddenly I forget what's coming up next... I'm sure, many musicians share the same or similar memories and have many stories to tell.
Now after many years of variuos musical practices I realized that the main problem of me having this horrible feeling about lost memory was that I wasn't well prepared. Of course, at that time I couldn't be honest with myself as I can be now.
I always started to memorize my music too late, so I never had enough time to live with it and to feel comfortable during the actual performance. Of course I was raised as a professional, so I would still play it pretty much well, but that would cost a lot of my nerve cells.
So if you have to perform from your memory, start to memorize well ahead of time. Be honest with yourself and hopefully you will avoid my nightmares.
You can fool your listeners but you can't fool yourself.