There is nothing wrong with wanting a more traditional style of relationship. It sounds like your girlfriend hasn't been quite honest with you and that isn't fair. I know you don't want to hurt her, but she did set herself, and more importantly you, up for a lot of hurt by not being forthcoming from the beginning. She has a fault in this, and it isn't your job to protect her from the consequences of that mistake.
I'm very sorry that your first introduction to the idea of poly is a situation like this.