I think the noun "cynics" and the adjective "cynical" are most appropriate as these are in common usage and one meaning is "doubtful as to whether something is worthwhile".

You have the adjective "fatalism", which is belief that the worst will happen. You can call somebody a "fatalist". This means they predict disaster.

There is also "catastrophising" but both of these are quote formal. They would only be used in the company of people skilled with language, and they are more predicting disaster rather than a low level of success.

Fretting is essentially worrying and does not necessarily imply they express their worries outwardly, while catastrophising is used to describe the outward expression of disaster scenarios.

While "pessimism" is not technically the correct term, this would also be used as it means the expression of negative expectation of success.

I think the noun "cynics" and the adjective "cynical" are most appropriate as these are in common usage and one meaning is "doubtful as to whether something is worthwhile".

Next you have "skeptic" and "skeptical". These imply the person has doubts, but it is a less conflicing, less combative doubt compared with cynic, which implies there is some underhand hostility.

You have the adjective "fatalism", which is belief that the worst will happen. You can call somebody a "fatalist". This means they predict disaster.

There is also "catastrophising" but both of these are quote formal. They would only be used in the company of people skilled with language, and they are more predicting disaster rather than a low level of success.

Fretting is essentially worrying and does not necessarily imply they express their worries outwardly, while catastrophising is used to describe the outward expression of disaster scenarios.

While "pessimism" is not technically the correct term, this would also be used as it means the expression of negative expectation of success.