I don't think it is "let" her go. She didn't ask you for permission. She is making her own choices right now. They aren't kind or good or honest, but they are hers.
You have your own choice. You can live a life that is kind and good and honest. You can go and make a scene (which is unlikely to result in her choosing you that night). You can invite her out (pursuit) or you can have your own plans where you will look and smell your best and do something nice for yourself (which might confuse her and make you look more interesting than the desperate spouse you've been showing for a while).
