Usually, very tells us "to a significant extent/degree" (in the opinion of the speaker).
On the other hand, so tells us "to such an extent/degree", and originally meant something like "in this manner". When used as you did ("She became so frustrated..."), there's usually a following phrase with "that" (a subordinate clause) explaining the extent.
A good example is a common joke you can sometimes hear on TV or comedy routines. Someone says "My brother is so stupid." and the audience yells "How stupid is he??". The joke finishes "He's so stupid that he showed his ID when he bought root beer.".
Sometimes so may be used in a phrase such as "That wasn't so hard.", again meaning "to such an extent". The meaning is like "That wasn't as hard as you thought.".
In modern speech people often say things like "I'm so tired." to mean "I'm very tired.", but there's a feeling of an unspoken situation which could be true because of how tired the speaker is, as in "I'm so tired that I could sleep while riding a roller coaster.".