Reticent
/ˈrɛtɪsənt/ adjective mediumInclined to stay quiet and keep one's thoughts, feelings, or plans to oneself.
Even after winning the award, she remained reticent about the personal struggles that led to her success.
Memory Trick
Notice the word 'silent' almost hiding inside the sound of 'reticent' -- a reticent person holds words back the way a dam holds back water.