Silla, I think you might be. For me, poly has two definition, the relationship orientation and the relationship dynamics. The orientation means being able to be in love with more than one person at the same time. The relationship means having relationships with two or more people who are aware of one another and fine with it.
It seems to me you might qualify for the first one. Now the thing is, even if you out yourself to your boyfriend, it doesn't mean you have to do anything about it. But if you keep it a secret from him, it might drive you apart. Already when he says he loves you, your mind goes to negative thoughts of hurting him instead of happy thoughts of loving and being loved back.
If you tell him you don't plan on cheating on him, but you want him to know that you have developed feelings for others and you think you are poly (and explain to him what that is) and you want him to know that, and you love him and don't want to hurt him... I think that would be the thing to do.
In my experience, just talking about it, knowing it's not a secret anymore, that can help. Of course there is no knowing how he'll react but it seems to me he deserves a shot at making a decision there, and not have you do it without him. I think because it's an important part of you, it's the kind of thing he would want to know. That doesn't mean it will drive you apart.