Simon, those negative thoughts are a pain in the butt, aren't they? I find it helps to collect inspirational quotes that "speak" to you. There are plenty of good quotes, but the ones you want to save are the ones that feel right to you, the ones that that make you feel great about yourself and your potential. When you're having these thoughts, read these quotes, or even post them around your room to remind you of your dreams and abilities. Here's a few you might like:

"The best way to predict the future is to create it." - Sophia Bedford-Pierce

"Shoot for the moon. Even if you miss, you'll land among the stars." - Les Brown

"Our greatest glory is not in never falling, but in rising every time we fall." - Confucious

"If you can dream it, you can do it." - Walt Disney

"Obstacles are those frightful things you see when you take your eyes off your goal." - Henry Ford

"Live with passion!" - Anthony Robbins

"Our doubts are traitors, and make us lose the good we oft might win by fearing to attempt." - William Shakespeare