Though this is likely true in a lot of cases, it isn't always.
For example, we had this kid who was real ugly in HS, I mean as bad as it sounds, he literally looked like Igor. However, he participated in school events such as One Act and in student crowds cheering on the sports teams.
He went from a 0 when I first met him to at least an 8.

A lot of history shows that what you believe determines who you are. An example might be that those in ancient civilizations who believed they themselves were Gods rather than rulers became popular in society and did great (or notorious) things. Not only that, but psychologists have been saying this for decades, it's nothing new.

The 'fake it till you make it' saying it probably your best bet, but you've got to truly believe it.
Start meditating daily. Sleep with hypnosis affirmations on in the background. Listen to relaxing and peaceful music on your way to school. Watch TED videos on loving yourself.
There are hundreds of tools available to you online that you can use for believing in yourself.