how to tell my best friend I used to self harm?

[Leigh]

Well, I have been through this before, and I understand how you feel. It depends how comfortable you are around each other before you tell her. I'll give you an example scenario for two close friends:

You: Uh, [name]? I have to tell you something.
Friend: Sure, go ahead.
You: Well, about [ the time since you last self harmed], I was feeling a little down and I felt like anything.
Friend: Aw, you poor thing!
You: [smile] Well yeah, but I was so unhappy, that I did something. Something terrible, that I would never do again. I self harmed, [name].

And hopefully, this will bring out a positive vibe during the conversation. Smiling helps, by making your friend more comfortable. And saying her name often keeps her engaged in what you are saying. I hope all goes well.