No pessimist ever discovered the secrets of the stars, or sailed to an uncharted land, or opened a new heaven to the human spirit
Keep away from people who try to belittle your ambitions. Small people always do that, but the really great make you feel that you, too, can become great.
Surround yourself with positive influences, and avoid the negative. Your mind is to precious to allow other's negativity to invade it. Seek out opportunities to find positive people, and people who support your ambitions. Drop out of the "ain't it awful club", as Jack Canfield calls it. Spend some time each day reading or listening to inspirational material, this will help counteract the negativity you inevitably run into daily. Also look for opportunities to socialize and work with positive people. Success and failure are both contagious, make sure you catch the right one by surrounding yourself with the right people.