My bf wants to be poly and I’m monogamous

Hi so I'll just get to it. Idk what to do or how to feel. My bf has been friends with his ex from years back. He's still kinda in love with her and always has been. Anyways he's always kinda wanted another gf (for us to have all a relationship). He brought up he wants to be a throuple with her. I don't want her. I don't want him to be with her. But I want to be with him and not give him up. I brought up him and her having their own separate relationship from ours (even though it's not what I want. I just don't wanna be selfish and I don't wanna give him up. Like he's poly and I don't feel like I alone am enough to fill his heart). He says he thinks it wouldn't work and would be too much trouble/someone would end up getting hurt. I'm already hurt and I feel like she's always been it for him and I'm just.... here holding him back from true happiness. Idk he says I'm enough and he loves me. But when he won't give her up and he wants to be poly so bad in general. I just can't really believe him...... idk what to do or how to feel. I'm destroyed right now. Please any poly people weigh in and help me....