How do I stop bein her friend without getting grounded?

Ok so theres this girl lets call her Cindy. So Cindy and I were BFF's sicne 3rd grade but the problem is we are in 8th grade and she is LESS mature then she was in 3rd grade shes super embarrising like there was this guy I liked and she went and embarressed me in front of him and he was cute AND he liked me well not anymore so one day this year I told her how I felt in a calm gentle way. She started crying then when she got home she told her mom and her mom said that if I wasnt her friend she was going to tell my mom I made her cry (even though I wasnt being mean) and if my mom heard that boom I would be grounded for at least a month then I would be forced to be her friend. So then I acted like I was her friend even though she doesnt use deo and thinks your so mature if you have public hair she is So stupid like when I'm with her or think about her I want to punch her in the face. She gave me a christmas present and it was a stupid hand bag I could hardly fit my phone in and ok if she bought it from a store and put thought in it I would have been happy well turns out what she did was her mommy and dadda (thats what she calls her parents) found an old handbag in their basement wrapped it up then handed it to her to give to me another time I was in a club and asked her if she wanted to join she said she did then got all excitted about it then after 2 mo she quit you know why cause she didnt like the meetings where we sat for only 15 mins then wedid fun activities she such a crybaby and a idiot one time I told her not to tell anyone somthing she went home an hour later and told her mom and dad and sister and her sister told everyone so not a secret anymore so my question is how do I stop being her friend without my mom knowing I cant talk to my mom about it because if I bring up anything about not being her friend my mom will ground me :(