Why does Juliet decide to go see Friar Lawrence at the end of act 3?

The Nurse has been Juliet's lifelong confidant as her nursemaid in infancy and now her sole counselor during her teens. Juliet is closer to Nurse than to her own mother, with whom she has a more formal relationship. Fearing for Juliet's state of mind and knowing the Capulets would never sanction their daughter's marriage to a Montague, the Nurse has daringly circumvented their wishes by helping Juliet marry Romeo in secret. Juliet has appealed in vain to her parents to forestall a union with Paris, so she turns to the Nurse for remedy. Knowing Romeo has been banished and Juliet risks being disowned by her father, the Nurse feels it would be in the girl's best interest to forget Romeo and marry Paris, according to her parents' directives. 

Romeo is banish'd and all the world to nothing

That he dares ne'er come back to challenge you;

Or, if he do, it needs must be by stealth.

Then, since the case so stands as now it doth,

I think it best you married with the county.

O, he's a lovely gentleman!

Romeo's a dishclout to him: an eagle, madam.

At this point, Juliet feels abandoned by the one person she has trusted the most. Out of desperation, she turns to the Friar as her last hope. If he cannot give her the help she seeks, she determines to kill herself. 

Ancient damnation! O most wicked fiend!

Is it more sin to wish me thus forsworn,

Or to dispraise my lord with that same tongue

Which she hath praised him with above compare

So many thousand times? Go, counsellor;

Thou and my bosom henceforth shall be twain.

I'll to the friar, to know his remedy:

If all else fail, myself have power to die.