DIFFIDENT

reserved, shy, unassuming, lacking in self-confidence

/showing modest reserve

lacking self-confidence
SYN - shy, timid, unsure

//hesitant in acting or speaking through lack of self-confidence
SYN - reserved, unassertive

"Although it seemed to almost allways be considered a fault in today's world, it was this diffident attitude of hers that made her so attractive to the equally reserved johnny."

"Brad hated his diffident nature. If there was one thing that was holding him back from success it was his lack of self-confidence and his timid nature."

"Unlike the decisive and quick-to-act Cathy, Kit was rather diffident."