Hey lovely,
I know it probably feels very hard to say those words. I think you've got to be fairly confident that she will understand - it can be pretty bad to have someone react badly. That said, the majority of the time people will only react badly because they don't understand it. Maybe you could print off an article about self harm - help her understand it better? If you can't actually say those words, maybe you could write it down, tell her you need to talk to her about something important and it's really hard for you to say it and then hand her the note?
Best of luck and take care
