What can I do for my best friend for 4 years to forgive me? PLEASE HELP!!?

My best friend and I have been through almost everything together! But this problem we recently had, we didn't come out strong. She was dating this guy for about 1 year on and off. This summer she got back together with him and this relationship is wrong. I didn't approve of it, because he didn't treat her right and I feel like all he wanted was sex. I told her mother and sister everything she has done because I thought it would help her get off the bad road she was going down with this guy. But it went horribly wrong! She screamed and yelled at me and said I back stabbed her. I know I should have told her what my feelings were about her boyfriend but I thought she would have gotten mad. Now I'm stuck without a best friend. I'm going crazy at home! Someone please tell me what I should do. Call her tell her how I feel? Write a letter? Or what?!