The journal is one way of freeing yourself from the misguided belief that 'Writing Should be Important and For a Lofty Purpose' and remind yourself that 'writing about nothing is always worth doing'.
You will find that it will supplement and stimulate all of your writing. Never stop keeping a journal, even when busy with other work. It will always be a source of inspiration.
We are haunted by our childhoods, by the important things we lost on the long walk to adulthood: the intensity of love and fear, the talismanic rituals and objects of affection, and the moments of comprehension of our place in the scheme of things.
What your character thinks is shown in everything she says. She may be proving or disproving some particular point or enunciating some universal proposition. All these comments reveal her personality. Her remarks also show her moral purpose. There are things she wants to talk about or things she wishes to avoid.
Don’t confuse the reader by having your character talk about something that has absolutely no relevance to the story. Everything said must be relevant, revealing the character’s motivation, experience and attitude.
Story is built from your characters’ goals and ambitions changed by circumstance and connected by one unifying theme.