FA has it right, all she wants is a reaction, this validates her feelings for OM, don't give it to her. Drop the flowers and the talks, affection. Just be happy and act "as if". If she thinks she has lost you, she will think twice. You don't stay with someone for all that time and not love them, she is infatuated with this idiot and it won't last, but it will take some big time, so sit back and relax and get ready for the coaster ride.
But when you react, you validate and drive her to him. When you show love for her, it reminds her that she doesn't love you, she loves him. So stop both of those now, they aren't and will not work.
