What is the ruling on writing down one's inner thoughts, such as describing sadness, pain, or joy?

There is no harm for a Muslim in speaking or writing about their illness or condition to a doctor or the like, as long as it does not involve objecting to Allah's decree and destiny. Imam Al-Bukhari cited a chapter titled "Chapter: What is permissible for a sick person to say: 'I am in pain,' or 'Oh, my head,' or 'My pain has intensified,'" and the saying of Ayyub (Job), peace be upon him: "Indeed, adversity has touched me, and You are the most merciful of the merciful." Complaining about Allah is not permissible. However, merely complaining is not blameworthy as long as it does not reach the level of discontent with what has been decreed. Therefore, it is permissible for a sick person to inform their friend or doctor about their condition, by agreement of the jurists.