Are all myths superstitions, or are all superstitions myths?

Myth in its proper sense is a very rich and deep word, meaning a reflection of truth. Myths can be true, and they are intended to convey truth even when they are not, strictly speaking, true. Myth is a means of communicating non-empirical truth in an oral culture.

Superstition can have its roots in mythology, but is usually about actions performed without a proper understanding of the reasons.

Both words can have both negative and positive implications.

“Myths,” CS Lewis told JRR Tolkien, were “lies and therefore worthless, even though breathed through silver.”

“No,” Tolkien replied. “They are not lies.”

A myth is an echo of temporally distant events remolded into archetypes easily transmitted through oral histories. Myths are closely related to legends.

A superstition is “an unreasoning awe or fear of something unknown, mysterious, or imaginary, especially in connexion with religion.” [OED]

The two words need not intersect.

These two words are not interchangeable.

A myth is "any invented story, idea, or concept"

A superstition is "a belief or notion, not based on reason or knowledge, in or of the ominous significance of a particular thing, circumstance, occurrence, proceeding, or the like."

So the two words are definitely not interchangeable.

"Elephants are scared of mice" is a myth.

"Walking under a ladder brings bad luck" is a superstition.

A superstition is usually a belief that some event will bring either bad or good luck. In the broadest possible sense, a superstition could be considered a type of myth, but not vice versa.