Diffident

dif·fi·dent /ˈdifidənt/

Adjective: Lacking confidence in one’s own ability, worth, or fitness; timid; shy.

Synonyms: modest – timid – bashful – sheepish – unconfident – embarassed -shy.

Quote: “A true sceptic will be diffident of his philosophical doubts as well as of his philosophical conviction; and will never refuse any innocent satisfaction, which offers itself, upon account of either of them.”
-David Hume