I think i made a big mistakee!?

so .. me and one of my best guy friends have known eachother for like 12 yrs... nd him and his girlfriend were goin ou and in that time i became really close with her and i trusted this girl with everythingg.. they were going out for like a year and half.. almost 2 years and i knew her for bout 6 months and became super close in that time.. one night my best guy frined went out to a party and cheated on his girlfriend.. when he was asked bout it he denied it.. but word was spreading fast and i was at the party with him and i knew that it was tru.. i didnt know what to do and i might be the worst lier EVERRR.. and so i couldnt hold it in any longer.. i told his girlfriend, and he said that he swore on her life that he never did it.. then it came down to that she asked me why i would lie about something like that and honestly.. its no a lie so i told her.. they broke up and now my best friend of 12 years wont talk to me.. what should i do? was i terribly wrong? i dont want him to hate me