I would be very surprised if _any_ astronaut feels assured they will come home. I'm sure they don't talk about it much, but there's got to be that little voice telling them there's a chance...
History is full of people knowingly taking such risks.
For example, my father was a B17 navigator. 80% of the group he went to England with became casualties. He was convinced he would not survive his tour due to the inevitable mathematics of the B-17 loss rate.
That is an interesting thing to think about. By contrast, I witnessed a young person who was afraid to pull over to the side of the road to help a stranger with a broken down car. I know these two are not a perfect comparison, but I can't help but think our culture has changed since then. That's just intuition -- could be right or wrong.
I just read that Glenn flew 149 combat missions, including ones where he got his plane back home all shot up. He had no shortage of bravery and willingness to take huge risks.
John Glenn was known to his flying buddies as 'old iron ass' for his ability to always bring back a shot up plane safely. He took heavy hits to his aircraft a dozen times.
He kept a Cessna, flying it until his 90th birthday.
And yet, they still go.