beside oneself

(verb) to be extremely nervous, angry, or worried

Example Sentence:

I am beside myself because I have a test to do today.

The teacher was beside herself when she caught her students cheating on a test.

I know you saw your boyfriend out with another woman, but you need to calm down. There’s no reason to be beside yourself, it may just be his sister!

Notes:

beside oneself is often used with the preposition with plus a noun form:

She was beside herself with nervousness.
He is beside himself with anger.
They were beside themselves with worry.