Nikki - I have never posted on MN before but I've joined tonight to try and offer you some advice.
FYI your story rings so many bells with me. My DD went "off the rails" aged 15yrs. She is now 17yrs and controls us with regular SH episodes, binge eating/laxatives/vomiting and horrible irrational behaviour.
She is medicate for depression and under the care of CAMHS, she has a counsellor and sees a psychiatrist.
How do we deal with it????? Hour by hour is the honest answer.
Once a child starts showing such behaviour traits it's important to keep a level head and ensure they know that you love them but not their behaviour.
You say that your DD is "threatening to cut herself". Ask yourself "why is she making the threats - why doesn't she just cut herself if that's what she wants??".
We've learnt that the threats are used to control the parents - sometimes it's easy to give in, after all what parent wouldn't if it would prevent a DC from self harming? We used to give in but then realised by doing so DD was refusing to grow up and deal with issues. Maybe we were wrong in refusing to give in, after all she now SHs regularly - and what do we do? The answer may sound awful but we give her antiseptic wipes, sudocrem and try to ensure she has clean blades - we also cuddle and reassure WHEN she lets us. We love her unconditionally but SHing is her way of dealing with issues - it's a release in the same way a smoker smokes or a drinker drinks.
We hope to God that she'll get stronger and come out the other side but, until then we tell her we love her and scrub blood off the bed sheets every few nights. We also take her to CAMHS sessions and ensure she regularry takes her AD meds.
PLEASE PLEASE PLEASE don't think that I'm posting this to upset you - I just want to be honest about controlling teenagers and how things can escalate. Your DD is already scratching herself - cutting may become the next step. Oh yes, another thing, don't think that hiding blades will solve the problem - been there, tried it!!! DD will cut with pencil sharpener blades, broken glass even the plastic end of a shoelace if that's all that's available.
My heart goes out to you and your family, I hope things don't escalate BUT, if they do, be prepared for a rocky road and keep posting xx