Backup is right. You gotta be assertive if you want her self respect. You need to show her this is unacceptable, disrespectful to you and your relationship and you will not tolerate it. Break up with her. Tell her your not gonna be her puppet on a string waiting for her to tell you whats next. If shes got feelings for someone else then it IS her fault for putting herself in this situation and allowing herself to emotionally cheat on you by getting too close to him.
Believe me she knew when they first started hanging out and flirting that she was crossing a line and she didnt care about your feelings then and doesnt now.
If you want any hope of saving this relationship then dump her, wait for her to come running to you and let her beg for forgiveness. Let her no if she ever crosses a line like this again your not coming back.
Personally i dont believe in second chances so id just dump her and move on but its your choice
Sent from my GT-I9505 using Tapatalk
"Don't ask a question if you can't handle the answer".