About 5 years ago when I was in thick of my situation there was another poster here who I became really good friends with outside of the board. We are still friends to this day and text with each other often.
She could not let go of her ex-husband. She continued to sleep with him even when he was living with OW. She did this for 4 years hoping it would draw him back in. She became the OW and he happily took advantage of the situation. There was absolutely nothing I could do to talk sense into her so I just let her live her life how she saw fit.
About a year ago, she became bored of the situation and decided to move on with her life. She blocked him and went NC. About 3 months later, he became obsessed with her. He even dumped his fiance as a sign of his commitment. By then it was too late, and she had lost all feelings for him. But he does still check in every few weeks and ask her out, to no avail.
It pretty ironic how when you truly let them go (and mean it), they often come back around.