For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Louisa agrees to marry Bounderby, even though she does not love him, and rejects Sissy's friendship
Act II
The bank robbery is discovered; Louisa agrees to meet Harthouse for a tryst but instead shows up at her father's house, tells him everything, and collapses
Act III
Tom turns out to be the thief and is shipped abroad; Louisa separates from Bounderby and tries to rebuild her life and mind