WordNet sense 1 (people who are fearful and cautious):

cautious

WordNet sense 2 (lacking self-confidence):

diffident, unsure

WordNet sense 3 (lacking or marked by a lack of confidence):

unconfident

WordNet sense 4 (lacking conviction or boldness or courage):

faint, fainthearted, faint-hearted

WordNet sense 5 (lacking courage; ignobly timid and faint-hearted):

cowardly, fearful

WordNet sense 6 (easily startled or frightened):

shy

WordNet sense 7 (made timid or fearful as by threats):

intimidated

WordNet sense 8 (timid by nature or revealing timidity):

timorous

WordNet sense 9 (modestly or warily rejecting approaches or overtures):

coy

WordNet sense 10 (self-consciously timid):

bashful