My once peppy, happy fun loving, and
Hugs to you, this has to be so hard!
My DD who is now 19 had friends who cut when they were freshmen or sophomores in high school, and with therapy, yes they did get better.
Please trust your therapist on this and read as much as you can about cutting. It's not a precursor to suicide or an indicator of serious mental illness; it's a coping mechanism, most often used by high-achieving, perfectionist girls, to release some of the intense pressure they feel. As strange as it sounds, cutting gives them a relief from emotional pain, the way a runner's high gives relief from physical pain. She will outgrow this with her therapist's help, but she has to trust the person. (One characteristic of a cutter is that she often doesn't feel she can trust anyone with her intense emotions, not even the people she calls her friends.) If your daughter is talking to her therapist, this is HUGE, and she should continue to go. Don't press her about the cutting. Just mention casually that it's important to use hygienic implements because the real danger is infection.
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I am so sorry you are going thru this.