Toward the end of the play, Juliet is affected by her desperation. It drives her to visit Friar Lawrence to ask for his help, and to consider suicide a better option than marrying Paris. Her desperation to remain faithful to Romeo overwhelms her fear of faking her own death with the friar's concoction.
Moreover, a sense of righteousness in her purpose affects Juliet who has, so far, been relatively obedient to her parents, and renders her capable of defying and deceiving them. Returned from the friar's cell, she tells her father, "Henceforward, I am ever ruled by you" (4.2.23). We know this to be a lie, and Lord Capulet believes it.
Likewise, Juliet is affected by her fear. After her mother and nurse have left her for the night, she says that she feels "a faint cold fear [that] thrills through [her] veins" (4.3.16). She worries that the potion will not work or that it is really a poison that will kill her. She fears what it will be like to wake up in the vault with all her dead ancestors, or that she will wake up early and be trapped there without air so that she dies entombed. However, her courage and desperation spur her onward.
In the end, Juliet's sense of commitment and her faithfulness to Romeo affect her, compelling her to take her own life rather than live without him.