"The Years That Never Were" curse and "A Road Not Taken" are similar, it's true. The difference lies in the fact that "A Road Not Taken" is a decision that the character did or did not make whereas "The Years That Never Were" result from a force outside of the character's control.

Like say, in Disney's Beauty and the Beast, Belle flees the castle and does not get stopped by wolves. That would be a "The Years That Never Were" moment because then she would have never saw the kinder side to the Beast. "A Road Not Taken" scenario would be if Belle married Gaston. No one ruins a woman's future like Gaston.