Someone help me

My boyfriend has a female best friend and I hated it. I recently got to know her and I like her and she assured me that nothing would ever happen between my man and her. She even has her own relationship to worry about. It calmed me down but I can’t help but be afraid that something will happen. I know I can’t control him but I honestly can’t help but feel like I want him to cut her off. I know it’s not gonna happen but idk what to do. I’m trying so hard but I can’t help myself from having these thoughts. My therapist says it’s probably trust lacking in our relationship. I’m trying to be fair and allow them to be friends but I can’t help but feel deep down inside it’s gonna be so hard for me, when I feel it shouldn’t. Help!