How to Compette with an ex in a relationship

How to Cope when You Think Your Boyfriend Only Likes You Because You Look Like His Ex

Steps

Pay Attention to compliments. Are they all about the way you look? Or even a comparison such as "your hair is red like Sarah's, but prettier" or even "Sarah wore her hair the same way you do" could be an indication of whether he only likes you for your looks.
Get to know his ex better. If she goes to the same school, keep an eye out for her [but don't initiate a confrontation]. If you have no contact with her, scope out pictures [check if she has Myspace or Facebook or Bebo] and make sure if that really is the case.
Listen to him if he ever brings his ex up, or listen to his friends talk about her [don't ask them about her though] and try to get conclusive evidence/ confirm this.
Talk to a close friend about this [preferably someone who knows him through you] and see their reaction. If they think its definitely possible, you might be onto something here.
Having confirmed that he only likes you for the way you look, re-evaluate your relationship. Do you really want to be with someone like that? Sure if you guys break up you'll be single, but single also means available for someone outstanding, who actually likes you for the awesome person you are!
Try making him like you for yourself. Talk to him more, preferably over the phone so that you aren't faced with a situation where he wants to cuddle up and you want to have a deep conversation.
Share your dreams and ambitions, and make him realize that you're a different person from his ex, no matter how identical you both look.
Consider a slight change in the way you look if it REALLY bothers you. This can range from something as simple as a new hairstyle to a new haircut or even [in extreme cases] changing your hair color/ getting contacts. Before making any changes think about whether he's worth changing for.
Talk to him about it if you think it's getting out of hand. Don't be rude or confrontational, just bring it to his notice and consider taking a short break so that he misses YOU and not the way you look.