timid or afraid
pusillanimous - cowardly
A person who lacks courage and is afraid to act or speak out about things can be described as ‘pusillanimous'. The word has more negative connotations than others with a similar meaning as it also implies a form of contempt for the cowardice, which is why it is often used to express somebody letting fear stop them from doing something that they really ought to do.
Brian will not go far in such a tough, uncompromising business. He is too pusillanimous.
I pusillanimously took my time coming to a decision about whether to take the job or not, so by the time I decided I would the opportunity had gone.
Jane felt that the organization's failure to combat the claims made against it was due to the pusillanimity of its leadership.