Hey,
I know it seems hard but you can do it, she won't think your stupid - she is your best friend. You should talk to her about it so she can understand, people who don't understnad self harm may react badly to begin with, this is usually because they are worried and don't know what to say and do. Sit her down and explain, also ask her not to tell anyone; she should respect your wishes.
If you ever want to talk, feel free to
PM me,
Talk care and stay strong,
Charli :-)