Thank you for the suggestion.
Because most people come to a community like ours to engage in discussions and get support from others, we never decided to add a forum for people to just write down their own thoughts to themselves. There are so many websites and apps that allow you to journal (privately and publicly) that I feel like that activity is already very well covered by others.
We have allowed members to start a single journaling thread in their primary disorder forum that could continually update and post to, but most people stop posting to these threads after a few weeks. You're welcomed to do that if you want to journal here, but of course it is all public and others may comment on your posts.
We also have a much shorter journaling option built into the DailyMood menu option that allows you to write something (privately) next to the daily mood you choose. That is all saved so you can review in the future.