I️ need to stop feeling this way... help.

My fiancé and his ex recently started to get along and while this is great I️ can’t help but feel jealous. My fiancé hates her as she says. But she’s a really pretty nice looking woman. She’s fit and has a nice body on top of that she’s being extremely nice to my fiancé and he’s going right along with it. He’s being nice to her as well. They smile and laugh together and I’m just there like what is going on? I️ feel so jealous. He said tonight that he feels like she’s up to something because this suddenly happened overnight. But he’s going to take it to his advantage and ask to see his daughter a lot more since she would never really let him see her. Now all of a sudden she’s letting his daughter spend more nights. Before when he would ask, she would say they had to go to court since they have a court case for her custody. But now she’s no longer saying they have to go through court and she’s allowing him to see her when he wants. My fiancé is being really nice to her..... I️ need to stop feeling this way...