I don't understand, it doesn't seem fair...?

That was it, sorry to say this you sound like a very nice guy but maybe you were just way too nice? you know to the point where you got boring? Girls like to be chased, once they know they have you they loose interest. If she doesn't feel anything for you, just let her go if she realizes she wants she'll come back, if not Time will heal your pain. Believe me time helps a lot. At least for me it's that way.