This is definitely not the relationship you are looking for. Until she wants what you want, you will always be the one to get hurt. The longer you let this go on, the more hurtful it will become.
Wanting an exclusive relationship is not selfish. A lot of people want that. And wanting an open relationship isn't selfish. But the two are opposites, and not easily compatible.
I really don't think you should let this relationship go on any longer. There are a lot of girls who wants to be exclusive. You are emotionally investing into a relationship that will end up hurting you very deeply.

Best of luck with the tough decision you have to make.