What do you say when someone says you can do better than me?

Seeing as she is your girlfriend, you must see beautiful, amazing qualities in her. So when she says you can do better, let her know that you don't think there is anyone better. Tell her there are no eyes more beautiful, no smile more charming, no sense of humor cuter than hers, no woman with more grace and intelligence, etc. etc. Be honest, open, and romantic!

...But if you have to lie to say those things, maybe it's time to reevaluate why you are with her!