My best friend has been cutting whenever I cut, and she also cuts when I upset her

My Bestfriend (15f) and I (16m) have been friends for over 3 years now. Been through so much shit together. She just now told me she harms whenever I do. sh has been my way of coping with life, and depression. SH never hurt anyone except myself. It was a jolt that kept me from going fully numb.