"He lived in the shadow of Mary Jo Kopechne -'the young woman' mentioned above. She cost him the presidency."

Not precisely. Kennedy's actions cost him the Presidency. Not just his driving on July 18, 1969 when he was responsible for Kopechne's death, but his actions afterward, particularly his failure to report the accident right away and, more broadly, his drinking habit and playboy behaviors. No one can be sure now whether Kennedy was driving drunk, but no one can be sure of believing his statement that he was not, given his habit and his reputation. The public's lack of trust in him, and the public knowledge of his irresponsibility, is what cost him the Presidency.

All he achieved since that night could not erase what he did, and what he did not do, that night. Sorry, but he'll never rest in complete peace.