I cant stop thinking about his ex !?

I have been with my boyfriend 5 months now and i cant stop thinking abouit his ex! She gorgeous, always immactulate, thin, hair extensions, perfect tan...you catch my drift. Im...well..not. Dont get me wrong im not a total horror or anything and i can dress up and look nice when its required, but generally i think theres more to life than looking perfect 24/7. Im not thin, but not huge and my boyfriend says im beautiful the way i am. But i cant get her out of my head. And it doesnt help that until recently she wanted him back and pestered him non stop until he blocked her number. We are happy and i can honestly see a future for us, so why is she always on my mind? Any body else feel like this ?