We see from Juliet's closing speech in Act 3, Scene 5 that one thing she decides to do is cut herself off from her Nurse. Her Nurse has just told her that Paris is above Romeo and that she should marry Paris instead. Juliet now believes in her heart that Nurse is a liar. We see Juliet ask herself,

Is it more sin to wish me thus forsworn,
Or to dispraise my lord with that same tongue
Which she hath prais'd him above compare
So many thousand times? (247-250)

In other words, Juliet sees her Nurse as being unfaithful in her praise of Romeo, or a liar. Juliet believes that Nurse praised Romeo before and now with the "same tongue" is cursing him and saying he is beneath Paris.

This realization makes Juliet call Nurse a "most wicked fiend!" (246). It also makes her decide to cut herself off from her Nurse, as we see in her line, "Go, counsellor! / Thou and my bosom henceforth shall be twain" (251). In saying that their "bosom...shall be twain," she is saying that their hearts shall no longer be connected. In other words, their relationship shall from now on be broken. Hence, at the end of this scene Juliet decides to cut herself off from her relationship with her Nurse and to go to Friar Laurence for advice.  If he cannot advise her then she also decides that she could kill herself as a last resort.