I don’t believe in the existence of Bigfoot (or the Lochness Monster). They are just myths.
In the classical sense, myths are stories. Whether they are stories with a basis in fact is apart from the concept. But not in our culture. For us, to call something “mythical” is to imply that it is no more than a widely held belief with no factual basis.
I think most people would agree that (under our cultural definition) Sasquatch is a myth. There is no basis in fact for the belief that giant, hairy man-like creatures roam the Pacific Northwest, waiting to be featured in beef jerky commercials.
That man can discipline himself is another widely held belief.
However, unlike Sasquatch it is one one to which many do attribute a factual basis. For them, self-discipline is not a myth at all. It exists. For many years, I was among those believers, despite the fact that I could point to no evidence of it in my own life.
Commonly defined (meaning, I got it off the internet) self-discipline is the ability to control one’s feelings and overcome one’s weaknesses; the ability to pursue what one thinks is right despite temptations to abandon it.
By that definition, I now must admit that I don’t have a shred of self-discipline. That is not to say that I have no control over my emotions or that I abandon myself to temptation whenever it appears. Only that whatever discipline I do have has very little to do with SELF, with me. For me, discipline is exoskeletal, coming from the people who surround me.