I have been there with my DD now 19. I could have written your post word for word when she was 15/16. To start with I advise you to keep telling her how much you love her and that you will help her to get through this.
It seems impossible to be nice at times I know but if you do what I did and argue with her and get on her case all the time things will get worse. It is excruciating to watch and I feel for you.
I would try to get on her side. Tell her you are concerned about her, not cross, and get her to see her GP. My DD's behaviour was the same and I thought it was challenging teenage behaviour. Most teenagers challenge their parents but not to the extent that your DD is doing and my DD did.
Self harming is a very serious issue and she needs help. Your doctor can advise.
Try to find out if some thing has happened. Did she fall out with her friendship group, did anyone assault her, is someone putting pressure on her?
Talk to her school and see what they recommend. A supportive teacher or school counsellor can really make a difference. If close family can support her that can be helpful too and if so call them in to talk to her and spend time with her.
My DD told me that looking back she knew she was depressed from about 14 years of age. I really wish I had known that at the time.
Teenage girls can have poor body image and if you can encourage her to take up a martial art so something similar that could boost her confidence. Maybe spend a pampering day together with hair and nails etc to strengthen the bond between you. Anything you can do to keep her onside can only help.
So many young people are smoking weed nowadays. My DD did and it exacerbated her already emerging mental health problems. Perhaps the school can invite the police in to warn of the dangers to health and the impact that convictions can have on emoyment and travel opportunities.
It's a very tough time and the GP and school will give you good advice. If at all possible keep talking to her and try everything to keep her close. I spent years rowing with my DD and she threatened me and hit me a few times. You don't want things to escalate. Talk to everyone you can to get her help. Get counselling for yourself too.
I am conscious that there isn't one answer and I wish I could help you more.
There is light at the end of the tunnel. Myself and DD are great friends now. Best wishes.