There is one person whom you have no choice but to believe -- YOURSELF. Just to make sense of the world, you must believe what you say to yourself. To do otherwise would lead to insanity.
BUT, the problem is, not everything you tell yourself is in your best interest. Have you ever caught yourself saying "I'll never get that done" or "I'm just no good at this"? Now what is the point of saying something like that to yourself? THERE IS NO POINT ! It accomplishes absolutely nothing but to convince you of your own weakness. When you catch yourself doing this, STOP. And instead, tell yourself something like "I'm making good progress on that" or "I'm getting better and better at this."
Whatever you say to yourself, you'll believe. And whatever you believe, you'll be. That is the power of positive affirmations. They're not just "feel good" quote rituals, they actually program and re-program your mind for success and your destiny. After all, you are your most trusted advisor. Make sure you give yourself the very best support, advice and encouragement every day.