What are some examples in the play that provide evidence that everything is determined by Human nature and choice rather than fate?

Jason's explanation of his desertion of Media for a new wife is in his own eyes an act self determination, but everyone else knows that his attemp to change his fate is futile.

“Here I will prove that, first, it was a clever move. Secondly, a wise one, and finally, that I made it in your best interests and the children’s.”

In mythology, there is no "free will." Everything a character does is a direct result of what has been fated. For example, in Oedipus Rex, it is fated that he kill his father and marry his mother. His parents think to change that by ordering his murder as an infant, but it alters nothing, and he fulfills his destiny.

Media is driven to madness and acts of revenge. Actions in mythology are exactly as the gods will them. When Media goes to Corinth the play's chorus sings of the sadness of her "fate." What happens here is involves free choice in action obviously, but in the end........... all of the conclusions have alreay been determined. No matter what choices Media had made everything would have come out exactly the way it did in the end.