Journaling is an important part of this Kum Nye practice, helping you to integrate and understand the experiences, sensations and feelings which are often just beneath conscious awareness. It creates the opportunity for your body's wisdom to emerge.
You don't need to write at length, just a few minutes is enough. When you've finished writing, acknowledge what you have written, and then let it go. You don't need to 'vow to change' on the basis of what you've written. For now, accept it as information that you may or may act upon.
The practice of journaling here is simply saying what is so. When we tell ourselves the truth, without attachment or judgment, our consciousness has the opportunity to realign itself. This is a gradual process that doesn't need to be rushed by the ego mind, which tends to want to dispense with problems by attempting to fix them or shoving them in a closet.