General Synonyms:
* Shy
* Timid
* Hesitant
* Unconfident
* Bashful
* Self-conscious
* Modest (when referring to a lack of confidence rather than humility)
* Reserved
* Withdrawn
More Formal Synonyms:
* Apprehensive
* Doubtful
* Insecure
* Unassured
* Uncertain
* Self-effacing
Less Formal Synonyms:
* Nervous
* Awkward
* Unsure
* Jittery
Antonyms:
* Confident
* Bold
* Assertive
* Self-assured
* Outspoken
The best synonym to use depends on the specific context and the tone you want to convey.