Genetic Engineering. Always wanted to be one when I was a kid, went to college and got discouraged from that field because of my religion.
Then I actually grew up and realized that religion is just a tool invented by people to control other people, and also something that people use to cope with the fact that reality is unfair and unpredictable.
Anyways, career I have right now is pretty good, and I enjoy it a lot, but I do regret not becoming a scientist. At least I still keep up with a lot of scientific world and research as much as I can as a hobby.