The written word can highlight contradictions over time in our thoughts, while concealing logical contradictions in the present; in others, the opposite.
Keeping a journal is a simple way of tracking your thoughts and feelings over time and is something I’ve often aspired to do. My suggestions:
Keep it short, ambiguous, playful, poetic. But most importantly short.
Don’t try to write down what happened each day; your future self will probably find it as boring as you would another’s such account.
Keep it digital (if you are comfortable with such technology). Make sure it has some kind of online backup or sync, perhaps password protection and ideally reminders.