why would she do this

a few days ago i fount out that my best freind is going out with my x boyfreind after i told her that i still had feelings for him and she till went out with him i told her how bad she hurt me but she dosent even seem so care that she hurt me i realy dont know what to do i dont think that i can trust her anymore help me what should i do should i give her another chance or should i just let her go as a freind i dont know what to do i dont know why she would hurt me like this i thought that she was my freind