Should I Let Her Go or Get her Back?

Hi, I just go broke up with . We Have been close friends every since April but we just started going out November 29th and broke up Dec.12...She broke up with me because her ex....told her I did something that wasnt all the way true.....but 4 days ago she starts going back out with this ex.....Everyone is Telling her what she should do so I guess she's confused.But Im thinkin that she dosent care for me anymore cuz she brings him around in front of me like its nothing while knowing that I still care 4 her...She Brags on her new Boi friend as to hurt me....secretly it does.....when we used to go out she used to flirt with other ****** and everything but now she dosent do anything....but I dont knw if I should try toget her back or let her go....Oh I forgot to say that she comes to my house frequently (cheating should I say on her B*F)