Most Helpful Girl
I think you shouldn't forget that her pain was not directly caused by you but by the fact that you weren't communicating properly. I think it's not too late to fix this relationship, however it would need more attention from you and more understanding on her part. If she is very sad and hurt at the moment, this is a good time to be with her, listen to what she has to say without telling her that it will all be good in the end. Don't tell her she shouldn't be sad.
If she still has feelings for you, which it seems like she does because she is deeply upset (she wouldn't care to talk to you otherwise) try to help her find out WHY she is feeling the way she is, let her vent. It might be very hurtful to hear what she has to say but bear in mind that the sadder a girl feels the stronger words she will use to describe the feeling. Don't ask her how she feels, ask her why she feels the way she does.
Make sure you don't tell her that she should be happy. Let her be sad, just be there with her when she is and don't get frustrated if in the beginning it doesn't help.
Give her plenty of attention, reassurance. Let her know how you feel, what makes you sad, angry, happy.
Make sure it's all honest, don't pretend to care if you don't.
And when she is feeling better and when everything works out for you two, work on the communication.