They are not interchangeable. “Superstitution” usually refers to false beliefs that certain behaviors can prevent or cause good or bad things to happen, or that certain things predict that something good or bad is going to happen. Examples: If a black cat crosses your path, something bad will happen to your soon. If you break a mirror, it will bring you 7 years of bad luck. “Myth” usually refers to false stories about things that happened long ago. Example: In Ancient Greek myth, Icarus made wings out of feathers and wax, but when he flew too close to the sun the wax melted, and so he crashed and died. But “myth” can also refer to false present day beliefs. Example: It’s a myth that Country X supports human rights; in fact, it has a bad human rights record.

They are not interchangeable. A "superstition" refers to cause-and-effect, and almost always involves actions that bring good or bad luck. For example, it is a superstition that 13 is an unlucky number. "Knocking on wood" is a superstition; if you say something optimistic--"I think we'll win the game today--it is a superstition that your statement will "tempt fate" and bring bad luck, but you can prevent that by knocking with your hand on something made of wood. Many athletes are superstitious and have a special piece of clothing or something special they carry with them, to bring good luck. A "myth" is a traditional story, told as true, about history, or about the gods, or about how things came to be. Sometimes they contain some truth in them. Homer's Iliad and Odyssey are examples of Greek myths. Most cultures have creation myths that tell a story about how the world began. Many cultures have cosmological and astronomical myths--for example, Norse mythology said that lunar eclipses were caused by a wolf eating the moon. Real historical events may get simplified, distorted, and turned into mostly-fictitious stories. These are myths, too. A good example from US culture would be the story of George Washington and the cherry tree. I won't re-tell it, but it is a completely fictitious story. George Washington was a real historic person, but the story concerns an imaginary event that happened when he was a small child, and it is supposed to prove that he was very honest and never told a lie, even as a child

Both superstitions and myths are kinds of "widely believed, widely repeated nonsense." So if you want to say "that's nonsense, even though a lot of people say so," you can say either "that's a superstition" or "that's a myth." In this case, the words aren't being used with precision and are almost synonymous. Nevertheless, they still carry the shades of meaning I described. Consider "Don't ever turn your computer off, it's better to leave it on all the time." This could be called a superstition, because it's a cause-and-effect belief, and it advises you to do something in order to have good luck with your computer. Now consider "Mussolini made the trains run on time." That is better described as a myth. It has a kind of gods-and-superheroes aura to it, and it doesn't tell you something you can do that will bring you either good luck or bad luck. Because of the success of the television program "Mythbusters," there's been an increase in the use of the word "myth" to describe widely believed, widely repeated nonsense.