Panic attack, flying to South Africa from USA?

I've flown more than a few times and never had a problem. Until 3 years ago when I had a connecting flight, flying from Florida to Connecticut. That day was the first and last time I ever had a panic attack. On larger planes I've always been fine but for this connecting flight I had to change and get on a small plane, because the airport I was flying to was a small one. At first everything was fine. Then, BOOM out of nowhere severe turbulence. It was like the plane was struggling to stay in the air. And I was seated in the center of the back of the plane, i felt everything. My chest was pounding, I couldn t breathe, I was sweating and gasping for air. I literally started crying and praying. There was an older lady sitting next to me and we started holding hands and praying together. She was screaming and so was I. We eventually made it safely but I never got on another plane after that. January 2019 I'm flying to South Africa for my best friends wedding. Flight is booked but I'm literally terrified. And the total flight time is 14 hours. I'm not sure if I can handle it. I really want to go. This is my best friend since we were 2 years old. Anyone have any experience flying this long? Over the ocean. What should I expect? What can I do to prepare? I d love to hear your experience with flying long distance.

Delta airline basic flight question?

Ik you can't get your seat until 24 hours after flight but can you choose what seat you get of all the seats that are left or do they choose for you?

On delta airlines basic flight question?

On delta airlines basic flight ik you can't pick seat and is assigned to you 24 hours after flight but can you change your seat to any available seats left after check in?