The actual idiom is "Don't sweat the small stuff." Although it just so happens that "fret" has a similar sound as "sweat", it best to leave the idiom alone. Independently, you might say "Don't fret over ...", but don't consider that to be an idiom since the literal meaning defines it, unlike "Don't sweat the small stuff" whose actual meaning is difficult to infer from its literal meaning.

"fret over" ?

Yes, they are both the same. However, "Don't sweat the small stuff." is a bit more common.