Give an example of how Willy is his own worst enemy when it comes to trying to live the American Dream.

In the end, Miller constructs Willy as a human being to be
his own worst enemy.  Part of this comes from the fact that Willy is willingly crushed
by the weight of his own dreams.  His inability to understand the construct of the
various matrices that make it impossible for him to achieve success through the narrow
definitions he has allowed himself to embrace becomes the reason why his own state of
being makes him his own worst enemy.  In defining "success" through external means,
whether through money or status, Willy has aligned himself to be unhappy, to not be able
to to live out his own dream.  An example would be how he continues to live in the past
about what being a "salesman" used to be.  The idea of achieving respect from others,
connecting happiness to external means, and defining success not with subjective
criteria but through artificially established and external means makes Willy a person
who can never be happy because his dream, or his conception of it, will never be
achieved.  When he describes his existence as "ringing up a zero," Willy's definition of
happiness becomes external, something that makes him his own worst enemy for he accepts
what society says success ought to be as opposed to defining it for
himself.