My life has been filled with terrible misfortunes, most of which never happened.
—Michel de Montaigne
You had an excellent first sales call with your dream client. You agreed on the next steps, which would have advanced the opportunity for you and moved your dream client closer to her goal.
Then you received an email canceling your next meeting and asking if you could reschedule in a few weeks. The first thing your mind conjures up is that your dream client has lost interest. And you've lost your opportunity. Someone persuaded her not to move forward. It's over.
Take a deep breath. It's just as likely that your dream client simply got busy. It's very likely that something in her world needs her urgent attention, and as much as she'd like to move forward, she is distracted by this thing.