Without knowing the details of the situation, I can’t give any insightful and sage advice (not that I could otherwise, but still). Some questions you might ask yourself are (i) “why did she tell me” and (ii) “does she want me to do anything?” She may have told you it was a secret, but at some level wants you to do something about it (including telling someone).
As a standard boilerplate, suicidal thoughts, ongoing abuse, or anything else where serious harm (or potential for harm) can be averted by revealing what she told you, tell some one.
For those who live in the Seattle area and could use help overcoming a fear of needles (or dentistry), check out the Dental Fears Research Clinic, at the University of Washington. Some years ago, they did wonders for me.