When his medication stops working, a divorced writer must relive the inception of his MYSTERIOUS ILLNESS to remember how he stopped it as a child. (26)

The concept is unclear. The logline raises too many questions, not through intrigue but confusion.
What does mysterious illness mean? What are the symptoms? Is this a mental condition? How does it manifest itself?? How will he relive the inception of his condition? Where there no other people around him that remembered what he did as a kid to stop the illness?? If he did it as a kid why does he need to re live it again instead of fighting it as an adult? Is his goal to cure himself or something else?

Lastly, it sounds like most of the antagonistic force and MC actions are internal, in other words, it's not cinematic what action does he take that will be visual?

Think of Stephen King's IT. They stopped Pennywise and then suppressed the memory only to have Pennywise return and face IT all over again.

My story has a similar element. And it is a horror.

Just some clarification. If he stopped it when he was a child, why is he still taking medication?