I used to have thoughts like this all the time for a few years. Up until very recently, in fact.
But I can certainly relate to how it feels. I would always keep a "pissed off" expression in order to, I'm presuming, scare people away so I wouldn't have to interact with them. I thought I was hideous or looked like a retarded clown when I smiled, laughed, or even talked. So, I would rarely rarely speak.
I've come to realize I don't think I look that bad when I'm smiling or laughing anymore. It may have to do with exercise or my outlook on things changing, but I'm starting to let go of some of those nagging thoughts. I also had a couple of visible cavities and had those taken care of, so that definitely could've helped with the whole laughing and talking thing. But even before I had (visible) cavities, these types of thoughts were present.
Regardless, I found that the less I actually think about stuff like that, the less it worries me and the less I focus negatively on it. And the best way I can recommend "thinking about it less" is by occupying your thoughts with other things that interest you or make you happy, as cheesy as that sounds.
The thoughts are fleeting from me every day it seems, so surely there's hope for you.