In the US, many astronauts start as Air Force pilots.

And for the preternaturally calm and confident who don't have the perfect eyesight required to enter the Air Force, many of them apparently serve instead on nuclear submarines...

Jonny Kim was indeed a SEAL, and a few more things as well, with a CV almost as impressive as Johnny Sins:

> American NASA astronaut, physician, U.S. Navy officer, dual designated naval aviator and flight surgeon, and former Navy SEAL.

Note that "physician" here means Harvard MD.

The point of training someone to their breaking point is not to make them immune to breaking. It's to give them experience with a realistic battlefield situation and their own physiological responses during it so they stand a basic chance when it does occur.