WordNet sense 1 (lacking self-confidence):

diffident, shy, timid

WordNet sense 2 (lacking or marked by a lack of confidence):

unconfident

WordNet sense 3 (acting with uncertainty or hesitance or lack of confidence):

hesitant

WordNet sense 4 (fraught with uncertainty or doubt):

dubious

WordNet sense 5 (lacking or indicating lack of confidence or assurance):

uncertain, incertain