Like an elephant, Fanny has a very long memory. Also like an elephant, Fanny moves rather slowly. She is not large, however. There the analogy ends. At any rate, the past is crucial to Fanny and she often lets it dictate her present. She treasures old trinkets and judges people on their past conduct, often refusing to change her mind about them in the present. While Fanny doesn't want to embrace change, most of the other characters in Mansfield Park live in the present and try to ignore, forget, or revise the past. And nearly everyone in the book selectively recalls the past at times, deceiving themselves about what has actually occurred.
Fanny gives too much power to her past and to the past actions of others. As a result, she doesn't allow for changes in the present.
Mary has a healthy attitude about the past overall – she indulges in some memories but discards and moves on from others.