I too was a cutter, and started at the same age, started at 12, continued on and off through 15. (I'm now 34).
I had an N-ish mom.
As a teen, and younger, my parents did not allow me to have my own emotional space. They did not honor my emotional expressions. I did not know how voice my confused emotions. Additionally, they did not listen to me when I asserted what I wanted.
Cutting was a way of releasing otherwise unexpressable potboiler of emotional turmoil. Cutting was a private activity that asserts ownership of ones body, one's boundaries...Cutting, for me, was also a way for me to reenact how my boundaries were routinely violated by family members.
Think about it: your skin is your boundary with the world. The act of cutting onesself makes perfect sense if, psycologically, one feels like one doesn't have skin. The pain from cutting is a real reminder that I have a boundary, somewhere, and this time, I'm the one violating it...
Let her be who she is. This means, allow her emotional privacy. DON'T pry and ask her why she's doing it, she probably doesn't understand herself. DON'T ask her what she talks about with her therapist.
Instead, think about how concerned you are, and express that. NOT in a "Stop that you're hurting me" way - my mom tried that, it didn't do anything, it turned the conversation into being about her feelings.
Common sense points at the N-parent (what patterns of controlling/boundary violating behavor were there before? That set the stage for her to be in this place now...) but also other stressors in her life may be influencing her behavior. Part of it, for me, was also about dealing with all the powerful changes my body was going through, that nobody was discussing, in my family. Mom handed me a book and walked away. I felt totally alone and subject to everything my body was doing. Also the stress at school, and being treated as a sex object by boys and even adult men, was horrible for me. That was my stress. Your daughter's stress may be totally different.
My advice: Just spend non-pressured time together, talk about other stuff, hang out. Respect her boundaries as much as you can. Talk to your therapist about your fears of the consequences of your exN's behaviors on your child, so that you don't project your fears onto her. Let go.
You can't control how she's acting out. You can provide a nourishing, nurturing home environment where SHE can set the boundaries with her N-father, where her voice can be heard.
The cutting is just a symptom of something else. Work with the something else, and the cutting will go away.
Good luck and god bless...
Andromeda