I think Napolitana nailed it, as usual. Don't push, as much as you'd like to know. One of these days, when the time's right and she's feeling completely safe, she may share the full story with you. You can foster an environment in which she'll be more likely to feel safe, but she'll have to decide when--and whether--the time's right to share her story. Just love her, continue to be accepting and non-critical, and don't push on this or other topics.

Time, familiarity, and an accepting, loving, non-judgemental environment will be what are most likely to help her open up, assuming she can. Try not to look at it as if she's actively holding something back from you, either. It's far more likely that what she's actually doing is holding it back from herself and avoiding letting those painful feelings come to the surface. You just happen to be in the way.