Let me be blunt here. Sounds like your partner has a weak personality, and she will always be a "follower" and do what her friends do, just because she wants acceptance so badly. It also sounds like she has so many, or such deep problems, that she is easily drawn to self-harming behavior. This is what you have to look forward to for the rest of your relationship. Do you want more, or will you settle for this?
You are either ok with that, try to help her change (if she even wants to change), or find another gf.
I have a long time interest in psychology, specializing in relationship dynamics for 30 years.
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