Diffident
Etymology
From Latin diffidere, "to distrust," from dis- ("away, apart") and fidere ("to trust") — literally to trust the wrong way, i.e. not at all. Its early English sense was "distrustful," but the distrust turned inward: the diffident person mainly distrusts themselves. That is why it now names shyness rooted in a lack of self-assurance.
adjective · /DIF-ih-dunt/
Shy and lacking in self-confidence, hesitant to put oneself forward.
What it names
The quiet self-doubt of someone who has something worth saying but shrinks from claiming the floor.
Example
He was so diffident about his own work that he apologized for the painting before anyone had seen it.
Related idea
Modest is an accurate, unassuming view of one’s worth; diffident is modesty shading into self-doubt, holding back for fear of not measuring up.