So, think of something you admire in someone else you know and try to copy it. You can learn anything and practicing being cheerful or doing things anyway when you're scared or feel awkward is the only way to start feeling different; thinking can't change anything, just doing. You have to practice living and being.

Check these out:

http://health.discovery.com/centers/.../optimism.html

http://www.mcwilliams.com/books/books/ (Life 101 and Do It! are my favorites)