We become what we think. If you think of yourself as getting old and feeling tired most days, that's exactly what you'll be. It's all in your mind, you are who and what you THINK you are.
If you want to change your outlook, you have to change your way of thinking. Try: "I am younger than my years" and "I am full of energy."
Create positive affirmations to repeat regularly to help you see yourself in a different way, in a positive light. Keep all negative words such as 'don't' and 'not' out of your affirmations. Negative words negate the positive.
If you believe you are the way you are because "my parents were like this and therefore I am," you can also change this, whether it's a 'way of thinking' or a physical problem.
Create your affirmations, make them as succinct and easy to recall as possible. You could put them in your mobile/cell phone as daily Reminders or stick Post-It Notes on your fridge door. To repeat them once a day helps you to re-think your beliefs about yourself. To repeat them regularly changes your thought processes and your belief system.
I can recommend Louise Hay's book, You Can Heal Your Life, which provides lots of positive affirmations. Holographic Programming is the name given to this practice. It's about re-programming your thought patterns to become the person you want to be, or as Buddhism teaches - to re-train your mind.
Affirmations that are currently helping me to change are:
