This Is Our Youth, by Kenneth Lonergan

This is how the character Jessica is described in the stage directions:

She is a fairly cheerful but very nervous girl, whose self-taught method of coping with her nervousness consists of seeking out the nearest available oasis of self-assurance and entrenching herself there with a watchful defensiveness that sweeps away anything that might threaten to dislodge her, including her own chances at happiness and the opportunity of gaining a wider perspective on the world that might eventually make her less nervous to begin with.