Are the given sentences using 'does not exist,' 'those,' 'then,' and 'exist only in the mind' correct and acceptable?

Are the given sentences using 'does not exist,' 'those,' 'then,' and 'exist only in the mind' correct and acceptable?

Lorraine's personal conviction is that if she chooses not to believe in made-up scary things, then she will not be afraid of what does not exist. Why must she be scared of those things that exist only in the mind?