Insane obsession with an ex girlfriend

ok, this is long but worth reading cuz i think its a pretty unique situation.

i was with my ex (call him harry) for 3 years he was with his girlfriend before me for a year and a half. she cheated on him and broke up with him and he was really upset but he had always fancied me. we got together and at the begining she was still trying to talk to him and stuff, i wasnt having any of it, she tried talking to me as well. we ended up having a slagging match with each other which made me really angry as she started saying horrible things about him too.

thing is she was pretty, maybe prettier than i thought shed be. i started checking her facebook all the time nd looking at pix and comparing her to me. im not insecure about the way i look, im pretty too, but the thing is shes so different looking to me, im slim shes a bit heavy, im 5"5 shes really short maybe 5"0, she has blonde hair i have dark etc etc, im kinda rock n roll in the way i dress, shes really normal (you know, standard new look/topshop vibe for those of u from the uk).

So this is where it gets weird...since then, I have almost tried to be like her, at first cuz i didnt wanna bump into her and her think she was better than me. i havent even noticed im doing it but the way i dress has changed (which is partly normal cuz obviously getting older nd stuff), ive started dying my hair blonde, stuff like that. i wasnt even concious i was doing it. i created a fake facebook account and added her as a friend and check her page at least every day, look at her new pics etc..

it wasnt even that i was worried my boyfriend still loved her because we were incredibly close incredibly fast and i have never questioned how much he loves me. when we'd been together a month she went to his work and offered him sex and he turned it down flat so i know i can trust him. i dunno if this is the reason i hate her so much.. but yeah there was a bit of jealousy and insecurity at first, say in the first six months but since then its not that at all, its partly habit and boredom but i dont know there must be something else compelling me to do it cuz i know it isnt healthy.

the weirdest part is, me and my bf have been split for almost a year and i was the one who broke it off but i STILL do it! whenever something bad happens to her, like she splits with a bf, it just makes my day.

WHY AM I SO OBSESSED WITH THIS GIRL? i know this isnt normal but does anyone else have remotely similar stories to share, or advice? please comment cuz im kinda worried about myself, also be kind, im not crazy, this is out of character and i want to stop so calling me a freak and a stalker wont help, i know im out of order!