"The second-century Stoic sage Epictetus argued that "Your will needn't be affected by an incident unless you let it". In other words, we can be masters and not victims of fate because what we believe our capability to be determines the strength of that capability."

That is absolutely not what Epictetus meant. In fact, like other Stoics, he taught his students a technique called negative visualization that is literally the opposite of positive thinking. The author couldn't have picked a worse example of an ancient precursor to positive psychology.