How to tell your friend

How do I tell my best friend I self harm? I know it's not a happy topic and we usually are pretty lighthearted over text (which is where I plan to tell her as I'm texting her right now) but we do have little serious talks (we call them counselor sessions- we pretend to be that and ask each other questions about how we feel and stuff like that). I scratch my thighs with sharp things (like a thumb tack or safety pin) not hard enough until it bleeds, but enough to break skin and have it leave a mark. Yes I know it's bad and I'm trying to stop, but I need to tell someone. I feel like I'm losing my mind and I've changed into a completely different, less positive person. However due to trust issues in the past I have a hard time telling people things like this. She self harms as well, and because of that I feel like I'll make her more upset and I feel like I've failed her. I don't know how to tell her or how to bring it up. I'm terrible at things like this. Help?