Ophelia begins the play as a relatively happy go lucky girl, attached to the court through her father, but kills herself midway through the play, driven insane by the various powerful forces acting upon her.
I would look at the incident in which Polonius tells her that Hamlet cannot be so choosy as a man not the crown prince, that he cannot even choose which cut of meat to eat are the terms he uses, and so she cannot be so open to him, etc. Eventually he forbids her to have any further correspondence with him and she obeys. This forceful cessation of her love affair is one of the things that pushes her over the edge.
I would argue that likely the second is the "get thee to a nunnery speech" from Hamlet after she returns to him "remembrances" according to her father's orders. She still loves Hamlet but he appears to be absolutely mad. His rage at her for something she had little control over is perplexing at least and maddening at worst.
Her father's death at Hamlet's hand is the final event that appears to send her completely into madness.