Why was she leaving her job?

I assumed the whole incident inspired her to make a change. I assumed she was going to go to Italy. I don't think she had reason to believe that Jack would come after her or that the family would forgive her. Glad he showed up before she left.

I like to think that after calling off the wedding she decided to take a move to change her life by herself. Assuming that she'd continue to be alone, maybe she would travel by herself or look for another and more fulfilling job. When I was young I dreamed about meeting the love of my life and traveling around the world. As it never happened, I simply decided: why not seeing the world by myself? :-)

The job would also remind her everyday about a family she almost had. If she stayed in the job she'd have to see Peter everyday. Also Jack shamed her into doing something with her life, after she came down on him.