I know that to be beside oneself with worry is idiomatic. But it looks like to be beyond oneself with worry, anger, etc. is also used - I see quite a few results on Google Books. However, I have been unable to find a dictionary entry about the expression. So, is the phrase idiomatic after all?
The thing about idioms is they don't bend very well - they tend to break.
"Beside oneself" is an idiom. It has a very specific meaning -'overcome with worry, grief, or anger; distraught' - originally translated by William Caxton in The Aenid, from the French.
In the work, dedicated to King Henry VII’s son, Arthur, Caxton translates the French phrase, “hors de soi” (meaning “outside herself”) to “mad & beside herself” (with regard to Dido’s mental state when she learned of Aeneas’ departure), marking the first time the expression is used in print.
Trying to replace 'beside' with 'beyond' changes the inferred meaning completely. The listener would consider 'beyond' to be a limit not a mental state.
To be beyond one's ability or resources - implying something unreachable.