One thing I would add—Writing a gratitude journal once a week has been shown to be more effective than doing it daily (Sonja Lyubomirsky 2007)
I found two publications by Lyubomirsky in 2007. Only one contained something remotely similar to your claim:
Tkach (2005) demonstrated that participants who were randomly assigned to vary the types of kind acts they would perform on a weekly basis showed higher levels of happiness and well- being 10 weeks later relative to those who did not vary their kind acts, and relative to comparison controls.
However, this is about “kind acts”, not about a “happyness journal”.
Can you clarify the daily-weekly claim?
Sounds somewhat like exploiting the Fundamental Attribution Error. Other people (including imagined selves) are not that much influenced by external situational factors in our minds. Thus, they act more consistent with their internal characteristics. ActualMe is always thrown around by external forces and emotion chaos.