Rob,
It is very normal to have these thoughts, I think all of us did at some point. I was not a career changer, but I was a 22 year old kid thinking: “what have I done? I just took out a huge loan with no obvious way of paying it back”. Of course it all worked out in the end.
Tory wrote a great post on what it takes for an ATP student to be successful, check it out: What does an ATP student need to do to be successful?
Chris