how do i tell my friend

Okay so i have recently split from my bestfriends because they made me feel shit. And the other day they all ganged up on me at once and it made me feel so shit that i cut myslef. I havent donne that in over year and I promised someone i wouldnt. I have now broken that promise and feel guilty about that and still havent told anyone. My boyfriend knows and im showing him tonight because i feel like im hiding it from him but people keep finding out when getting changed from sports in school.