When someone cannot take in your reassurance while they are frightened, that is the receiver and not their willingness. Wait for the wave to pass and try again.

What the research says

There is good evidence that perception of the body's internal state can be altered by a condition rather than by attitude: a systematic review of chronic pain found altered interoception, and carefully separated accuracy, self-reported sensitivity and awareness of one's own awareness as different things. That is a different domain from hearing under threat, so it is a parallel rather than direct support. The general point holds: what gets through is not a measure of willingness.

Strength of evidence: one line of research. A single study, or a theoretical account.