Questions with Barthes Mythologies

While reading the section of mythologies based on Myth, and listening to the professor's lecture i was met with a few problems in my understanding of the concept of myth.
First off, the idea of "defending" against the creation of myth was difficult for me to grasp. Barthes postulated that an "artificial myth" staves off the creation of unique myth, but why would that be so? If myth is truly like "flowing water" and able to surpass any human intention then wouldnt the act of creating a myth intentionally make the basis for a new myth? By the same token that the political campaign ads myths were based on the desires of those creating them, wouldnt the act of making an artificial myth show the intentions of its original creator? Perhaps there is something that I am not getting, but even the answer during the lecture was not particularly satisfying.
Again, I am feeling a little silly for not understanding this but I wanted to ask anyways, what is the difference between myth and metaphor? To me, it seems that the original sign (what becomes the form) is just a different way of saying that "this represents something else". While there seems to be something more nebulous within a myth, at least as described by Barthes, I cannot entirely grasp what the quality is.