Dude i know it sucks to lose someone who you care about but look at the behaviour she presented you with, a break for girls night is her wanting to have a one night stand and cheat without it being labeled as cheating. She didnt want to disappoint her friends but she is fine with disrespecting you and being rude to you. She throw the promise ring at you, thats not something that someone does if they love you, she knew what she was doing the whole time and choose to do it, its not a good idea to take her back. Its only a matter of time till this happens again and she is disrespectful again. It sucks but its best to dump her and move on. She probably did something on girls night she regrets but she will never tell you, dont take someone back who doesnt treat you proper