Hear something enough times and you will come to believe it, regardless of whether it's actually true.  It's called the illusion of truth effect.  A few common examples:

• Bulls are enraged by the color red  - they're not
• Lizzie Borden took an axe and gave her mother 40 whacks… – she was found not guilty
• Swallowed gum takes 7 years to pass through your system – nope
• Napoleon was short – he was average height

Also noteworthy that marketers, tabloids and politicians use this all the time.  They repeat something over and over again until we believe it.

Suppose that you told yourself repeatedly that you're a fat, no good, useless slob or that eating mashed potatoes with gravy would make you gain weight.  Eventually this feels true and you start to live your life under a faulty set of assumptions.

But what if you did the opposite?  Try telling yourself you are a beautiful and wonderful human being.  Do this until it feels true because eventually it will.  Imagine how your existence would be different if you actually felt this way about yourself.