Romeo and Juliet Act 3 Scene 5 How does Julliet react to the Nurse's advice? Why is she devastated?

The answers to your questions can be found in the following passage from Act 3 Scene 5

Nurse

    Faith, here it is.
    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,
    Hath not so green, so quick, so fair an eye
    As Paris hath. Beshrew my very heart,
    I think you are happy in this second match,
    For it excels your first: or if it did not,
    Your first is dead; or 'twere as good he were,
    As living here and you no use of him.

JULIET

    Speakest thou from thy heart?

Nurse

    And from my soul too;
    Or else beshrew them both.

JULIET

    Amen!

Nurse

    What?

JULIET

    Well, thou hast comforted me marvellous much.
    Go in: and tell my lady I am gone,
    Having displeased my father, to Laurence' cell,
    To make confession and to be absolved.

Nurse

    Marry, I will; and this is wisely done.

    Exit

JULIET

    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.

Juliet has just been ordered by her father to marry a man other than Romeo. Juliet protests, provoking her father's fury. Nurse defends Juliet from her father, but then advises Juliet to forget about Romeo, for he is "banish'd" and accept Paris is a "lovely gentleman." Nurse compares Romeo and Paris, declaring that Romeo's a "dishclout" to Paris.

Juliet states "Amen!" and pretends to accept Nurse's advice. Thanking her nurse for comforting her "marvellous much," Juliet is in fact tricking her nurse into thinking that she has accepted her betrothal to Paris, when in fact, on the inside, she is cursing Nurse for insulting Romeo. Cutting all ties with Nurse, Juliet seeks Friar Lawrence for a way to resolve her plight.