If they are leaving with someone else you don’t have a choice anyway. You want them to believe that they are not worth s*** to you. You do not want to be desperate or clingy if you want her back, that’s for sure. She can’t miss you if you are always visible and irritating. A pest. You don’t want her to look back and see a desperate loser clinging on wasting their life on someone who treats them like dirt. That’s a turn off even to me as a man if I am not interested in her.