What about arguing with the thoughts? For example, say you start playing "you're ugly" in your head. Snap back at it with a "no, I'm not" or something more specific even like "no, I've got a pretty face".

Typically whenever I get a compliment I either say or think to myself that the compliment is wrong... Very twisted, I gotta stop that. If I can do that for compliments, perhaps I can retrain my brain to do that for insults too. Might work for you?