It's safe to assume that is not the only way, because, self discipline without knowledge doesn't works.

I belive that not everyone have self discipline, but, they makes a good job and conquest a success in life, because the people have a different ways to live and work.

By analysing the Sanjay comments, her opinion makes sense. If promisse about second marshmallow is broken, the kids would fell betrayed, frustrated and the self discipline makes no sense anymore.