Boyfriend mag be Poly

My boyfriend and I have been dating for 2 1/2 years now and it has been lovely, But recently, I made friends with this girl, and they started hanging out one on one (which is fine by me. ) I never really cared because I had full trust in our relationship.

He recently told me that he caught feelings for her and at first I was shocked, but I never want to hold someone back from being who they are; so I said I’d let him try it, I let him know I have no romantical feelings for her at all, if more feelings come up let me know.

We got into this massive argument, and maybe I was in the wrong. Maybe I was in the right I really don’t know, but I told him I’m not comfortable with how far this has gotten so fast. For extra detail, he’s saying he has romantic feelings for her, and she has romantic feelings for him, but I don’t have romantic feelings for her at all and that’s not really how a poly relationship works.

Time passes and I’m slowly becoming OK with it …. UNTIL this morning… it was just eating me up inside. He said that he still has the romantic feelings for her and she still has those romantic feelings for me and I feel like a third wheel in my own relationship.

I told him I can only ever see me marrying you. I cannot marry another person. I thought I was gonna be OK with it. Turns out I’m not and I’m so sorry for letting you on but I only want you.

Mind you I’ve known this girl for about eight months and he’s known her for a month and a half and he states that he loves her just as much as he loves me, but he’s not going to choose between either one of us.

I told him if I had to choose, I know exactly who I’m choosing and it’s gonna be you not her because I love you and I know I wanna spend the rest of my life with you.

I don’t know what to do, any advice would help.