"Be diffident" means being hesitant or lacking self-confidence. If you wish to express this sentiment in a slightly different way, there are several synonyms you could consider. You might use the phrase "be uncertain," which implies a lack of confidence or assurance. Alternatively, you could use the word "shy" to describe someone who is hesitant or unsure of themselves when facing social situations. Another synonym for "be diffident" is "be apprehensive," as this conveys a sense of nervousness or anxiety about something. No matter which synonym you choose, each helps to express the idea of being hesitant or lacking self-assurance in different ways.