1. Let go of the assumption that the world is against you, or that you were born with a gray cloud over your head. It is an assumption that has no basis in reason or science. Sometimes we pick up a flair for pessimism from a parent who made negative assumptions about the world somewhere along the line. Either way, the sooner you can attribute your pessimism to a unique set of circumstances rather than the state of the world itself, the easier it'll be to change your perspective.
2. Understand that the past does not equal the future. Just because you've experienced pain or disappointment in the past does not guarantee that what starts badly will end badly. Do not make a bad start turn into a self fulfilling prophecy for a bad ending.
3. See yourself as a cause, not an effect. You don't have to be a product or a victim of your circumstances. Stop thinking about what is happening to you and start thinking about what you can make happen. If you're not happy with the way your life is now, set goals and move on. Use your past negative experiences to build character and make better decisions. Life involves taking many risks every day, and not all of them will end positively. That's what defines risk. But the flip side is that some actions will lead to good results, and it's generally better to have a mixed bag than to have nothing at all. Ideally, the good stuff will outweigh the bad, but you'll never reach that point unless you put yourself out there and hope.
4. Use positive affirmations. Write down short statements that remind you of what you're trying to change about the way you see the world. Put them in places where you'll see them every day, such as on your bathroom mirror, the inside of your locker, on your computer monitor, and even taped to your shower wall. Some affirmations to start with are:
"Anything is possible."
"My circumstances do not create me, I create my circumstances."
"The only thing I can control is my attitude towards life."
"I always have a choice."
"I choose to live my positive side of life."
5. Remember that life is short. When you feel pessimism clouding your judgment or you start to feel down about the future, remind yourself that every minute counts, and any time spent brooding guarantees nothing but less time to enjoy whatever life might have to offer. At its core, pessimism is impractical because it causes you to spend time dwelling on things that haven't happened yet and aren't guaranteed to happen, and it prevents you from getting things done. Pessimism breeds indecision. It's a waste of time, and time is a limited resource that you can't afford to take for granted.
6. Be a balanced optimist. Nobody's suggesting that you become an oblivious Pollyanna, pretending that nothing bad can or ever will happen. Doing so can lead to poor decisions and invites people to take advantage of you. Instead, be a rational optimist who takes the good with the bad, in hopes of the good ultimately outweighing the bad, and with the understanding that being pessimistic about everything accomplishes nothing. Prepare for the worst but hope for the best - the former makes you sensible, and the latter makes you an optimist.