Is it permissible for a girl to write down her feelings in her diary, including flirtation or admiration for a young man, as well as writing down a secret her friend told her, while being certain that no one will read what she has written?

The general principle is that there is no harm in writing down feelings, as long as it does not involve anything forbidden. However, writing down feelings for a young man reinforces attachment to him and can lead to the disease of عشق (passionate love) which requires treatment, not reinforcement. Writing itself is not a disclosure of secrets, but others might come across it, even after the death of the writer. A Muslim's time is precious and should be utilized for what is beneficial, for "Two blessings many people are deprived of: health and free time." The feet of the son of Adam will not move on the Day of Judgment until he is asked about his life, his youth, his wealth, and his knowledge. Adolescence means approaching puberty, not actual puberty, and it should not be taken as an excuse for corruption.