Self Limiting Beliefs
Your belief system is like an internal thermostat. It works just a like a thermostat in your house, if you were to open a door and it was cold outside, the temperature would drop inside. But soon after, the thermostat would kick back in and adjust back to the preset temperature. And if it was hot outside and you open a door the temperature would go up, but soon after it would cool back down to the preset temperature.
So just like your beliefs, you can adjust your behaviours a little bit, train hard, eat well and effect your attitudes a little bit or for a short time.
But if your internal thermostat is set permanently to a certain temperature (or to certain beliefs about yourself) you will always find yourself back there, no matter what you do on the outside. So if you find that you are a "yo-yoer", an "all or nothing", a "quitter", an "I'm gunna start on Monday or 1st Jan", "I'm big boned", "It's genetics", "I'm too old", "I can't (fill in blanks)" kind of person then maybe it's time you work on adjusting your 'thermostat'.