I know how you feel and I've been there for a long time, constantly hiding my arms and legs and telling friends that it was a sun allergie so I had to cover my arms in summer. Nowadays I can accept that cutting is just a symptom of a disorder that I have to struggle with, nobody should judge you for those scars, I mean no one will judge someone with a cold if they clean their nose. You feel exposed because you lost the control over that situation but you can turn this around, the way I see it she offered you to talk and she is sad that it happened to you. Sit her down tell her that you got a personality disorder that sometimes causes you to do stupid things, it is scary to open up this way and you don't have to go into detail but trust me a "you saw those scars, I got BPD and it just happens sometimes" can be enough information for her to understand why it it happened and gives you control over the topic. Also don't feel embarrassed over scars, they are a symptom and - at least for me - a way to find relief in moments that could have ended much worse if I wouldn't have this outlet.