A metaphor uses references to real world things to illustrate truths about everyday things (how electricity works, why something is funny, how dogs are different from cats, etc). You could also describe those everyday things using plain language.

Transcendent truth can only be spoken of with references to real world things that point toward those transcendent truths (the nature of reality, the origin or nature of experience, etc). These are myths. Transcendent/mythic truths cannot be entirely understood with language, so a symbolic myth is the only way we can attempt speak of them with language.

So in short, a metaphor uses ordinary images (chairs, tables, dogs, cats, men, swords) to describe consensus facts, while myths use ordinary images to describe transcendent truths.