She has moved on and she is having fun with the other guy and she wants validation from OP. If OP doesnt give her the validation she will paradoxically be more satisfied (because it will give her reason to chase) and behave better towards OP.
OP you need to be 100% confident she misses you and wants you and you must act arrogant. You are dealing with a bad girl that does not deserve your attention. If she wants your attention she is going to have to please you in any which way. Tell her you are going to give her punishment. It's tricky but that's the only viable option.
Then she will come to you. And if she doesn't, you didn't waste your time.