How come when you tell your friend thatll theyll be your BFF it dosnt happen??

she was my first friend moving to cali shortly becoming my bff and sister. we were always hanging out like normal bffs, just then one weekend out with the girls we partyd and i always babysat for her cause shes the one with the car and well i would never let anything happen to her the night when on like usal,then the next day the guys who threw the party before called us up and wanted us to kick it so we went it was like 8am so i was pretty hunged over..well they were nozing it and i just felt like puking so i watched her and she trusted me with her keys and phone so i took watch, i went out to take a breather and went back in couldnt find her and so i got mad took of with her car because i knew what vunerable thing she was doing and i ended up finding out what happen and well they took advantage of her but from there our friendship has been limited to hi byes,was it my fault was she depending on me? i dont know what to do its not the same i miss her its benn 3mths