diffident
adjective uk/ˈdɪf.ɪ.dənt/ us/ˈdɪf.ɪ.dənt/
羞怯的，怯生生的
shy and not confident of your abilities
a diffident manner 羞怯的举止
You shouldn't be so diffident about your achievements - you've done really well! 你不该对自己的成绩这么不自信——你做得的确很好！