I meant in public or at least that she attempts to project it. My point was that her confidence is an affectation she has adopted for political and personal convenience. It does not reflect how she feels underneath.
This one happen in a time of panic and not in public. We clearly see her try and regain her focus in a latter scene.
Jasnah is in private and Shallan catches her in a moment of weakness. By the end of the scene she has pulled herself back together.
Jasnah is actually quite confident and collected considering her circumstances. It is Ivory who is panicking. Jasnah is doing her best to save lives(including her own).
Remember madness in any form is a deep seated phobia of Jasnah's. She is also reading words promising the apocalypse. She is still doing her best to be analytical and draw useful conclusions. This is the soul of who Jasnah is as a person.