The actions of the nurse make me wonder how genuine she really was to Juliet. Back in those days would the nurse really be Juliet's mentor?
The nurse's job as a "wet nurse" was to breastfeed the baby, so as noble ladies like Lady Capulet didn't have to do it themselves. Of course, the nurse's child, Susan, was the same age as Juliet, but she has died:
Susan and she (God rest all Christian souls!)
Were of an age. Well, Susan is with God;
She was too good for me.
So Juliet becomes, in actuality as well as in personality, like the Nurse's surrogate daughter: far closer to her in the early part of the play than the icy Lady Capulet. The nurse takes the "cords" (the rope ladder) so that Romeo can climb to Juliet's window, and seems to be guarding them as they spend the night together in the aubade scene. Juliet has completely confided in the nurse about her marriage and her plans (some productions even allow the nurse to be a witness at the wedding!).
You're right though that the nurse eventually abandons Juliet - after Lord Capulet and Lady Capulet have left the room in the furious "God's bread" argument scene. Juliet turns to her nurse for advice, asking what she should do, and the nurse tells her
I think it best you married with the County...
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.
It's not - actually - the worst advice ever. But it doesn't recognise who Juliet loves. And from that moment, Juliet knows she has to "act... alone".