I am so sorry to read this :(
Eldest dd used to self harm when she hit senior school till in her teens and it breaks your heart to see it and you feel totally helpless as to what to do to take this away.
Hers was digging and scratching to finding a mirror broken in her room. Once she actually took apart a bic razor and I still have that nightmare picture in my head of what I saw. The difficulty was before she was self harming due to anxiety at school, low self esteem etc but then she hit hormones and teenage years and so it got worse when the teenager feelings of "you dont care" or "life stinks" etc homed in on top of her allready anxious feelings.
Only thing I can say is dont react in an emotional way. Talk to her definately! but remain calm and matter of fact. I used to plead with her not to do this to herself, ask her why etc but to see me distressed was infact a huge error on my part as it just adds to the guilt they already feel over doing it iyswim. They cant help it as its such an automatic reaction just like someone biting their nails (trivial example but equally becomes a habbit) They use it to block out whatever it is that is upsetting them.
The bathroom incident as I will call it but the lid on it. I broke dialled for emergency and by the time they came I was in a mess. After that I kind of went numb. She did that once more and this time I just went in, took her downstairs and got the first aid kit and patched her up. Calmly said "there all better now, DD we need to find something to help you stop feeling so sad" Mad her a drink and took her to bed and went to bed myself. (I stayed awake all night!)
Its sounds harsh but me having that "oh well sigh" but also the diplomatic caring mum (with hugs) sort of attitude actually did help as she didnt have to dread waking up in the morning to see my face or feel guilty which used to heighten the anxiety more. And I can honestly say she never did that again, but it took a while for it (the milder harming) to stop completely. Another thing I did was I got her to write it all out when she was angry or upset and rip it up as savage as poss and throw it in the bin. I bought stress balls and exercise stretch bands that she could pull and twist at home as a way to release tension another way instead of upon herself.
We used camhs and they were good with her in building up her confidence and self esteem, helping her with past family issues and sorting out the school which was the main hardship for her because they didnt understand the emotional pressure she was under at the time.