Astronauts must be nervous stepping onto the first manned flight of a new craft that has a 100% success rate in the sole previous flight, but might have only a 50% success rate by the end of their mission...

Apart from STS-1, we also have the even more recent/relevant SpaceX example of this happening: https://en.wikipedia.org/wiki/Crew_Dragon_Demo-2

With both Crew Dragon and Starship, there will have been _many_ successful missions involving un-crewed variants of the spacecraft (Falcon 9 and Cargo Dragon were both well-proven systems before crew was a possibility).

I don't think nervous is the right word. It's kind of the whole thing test pilots live for.

the word for nervous and happy is excited

Imagine being on STS-1, piloting a shuttle that had literally never been to space before in any form.

https://en.wikipedia.org/wiki/STS-1

Well at least they would have no one else to blame but themselves. As far as I remember it was the pilots that insisted on the shuttle to be not be 100% automated, so they had to do it this way. The soviets just made the whole shuttle automated so it could be tested without risk to crew.

Or similarly with Orion, which has never been to space with a fully functioning life support system, and will not be until it carries a crew.