just wanted to note that I had added a bit more after your comment, cuz I didn't know if it would get cut off. And you are more then welcome. Like I said, just because she is masterbating doesn't mean anyone showed her. It just means that it's a chance to find out and talk to her about it
No matter what happens I am more then sure you will be able to handle it. And I also think it's great that you are talking about it and grasping it a little bit more. What happened before with your response can be explained away and you both will get past that once you open the door to discussion. (In my opinion anyways, formed by your willingness to 'seek help')
Good luck!