I think you've got the answer in your post: why it bothers you is an old wound, and one that ultimately, you can't re-do. All you can do is change your feeling about it, and keep working at allowing yourself to feel differently through experiences in the present. But it takes time, and a willingness to focus your attention on the experiences/relationships that nourish you. It won't feel like enough, and it may prompt that need to feel cheated, wherever it comes from. But I think that each time you resist the familiar negative feelings, they lose a little power.

Why not block her from your FB? What keeps you from doing that?