that won't help. people can be self-aware through a number of ways. and unless you can measure the self-awareness of any one person and find people with a similar level of self-awareness then how will you measure the effect the journal has?
you need to compare your own self-awareness before and after the experiment. only that matters. you can do a study where you let a number of people start a journal and then measure if they report increased self-awareness. but depending on how self-aware they were before, the change may vary from a lot to no change at all without proving or refuting anything.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5114878/