shy and lacking confidence
diffident - shy
A person who is ‘diffident’ is reserved and unassertive. The word stresses a distrust in one’s own competence or ability which leads to hesitance and a fear of speaking up. Such a character trait can be very limiting in a professional environment, and also in a social one, where it may cause others to mistake your reserve for rudeness.
Paul was very diffident about his success. He wasn’t trying to be rude, he just didn’t want any fuss to be made over it.
Sally is very capable but her diffidence really holds her back. She’s always afraid to voice her opinion on anything.
He began his speech a little diffidently, but grew in confidence and authority as he went on.