let it happen, ask her to stop and ask her if she wants help to better herself. if she refuses and still does it, just end it dude. it’s such a mental exhaustion to have to deal with someone who won’t accept help
try to talk her out or distract her with a convo long enough for her to forget
I recall you saying it's long distance so just be with her and try to help persuade her to stop
keep the convo going and try to connect to new topics that she likes. the longer you two talk the more likely it is that she'll either forget or decide that she doesn't feel like cutting for a bit.
in all srsness you need to for now as others be saying distract her or take her mind away and put her in a better mood
and continue to follow this problem to the root. ask her things. try and get her to open up and try to help her heal the trauma that drives her to this.