adjectives - Is there a word that describes an individual who is always clueless when he has to face major challenges?

If a person does not have the ability to reason logically in high pressure situations (and is consequently always clueless), what term can be used to describe such a person? I'm looking for an adjective but a noun will also do.

If there is a medical term to describe such a condition I will be interested in knowing about that as well.

Example Sentence:

He sank into stormy contemplation, clueless how to handle the latest
of his challenges.

So from the above example, such a person can be called a __________