Obsessed/in love with all the girls my boyfriend cheated on me with?

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My boyfriend and I have been together almost five years. I seem to have an obsessive interest and attraction to not only his ex girlfriends, but two girls he has been unfaithful to me with - some of whom I have never met. This has been happening from the start.

I sexually fantasize about them, I stalk them regularly on social networking sites, essentially I have this weird, f**cked up attachment to them, like I can't let go of the past. The irony is I'm pretty ambivalent about my relationship with my boyfriend now. I used to love him more than I valued living, and I feel like part of this is residual trauma from when he cheated on me.

Clearly there's something wrong with me, but what is it? I must be hella repressed, because even I can't rationalise it. Also, may I add, apart from this one weird trait, I am a relatively normal, sane, functional nineteen year old girl. I'm just tired of this shameful, inexplicable baggage.