How do you think positively if you are a habitual negative thinker? How do you make the change!?

I only managed to become a positive thinker after many CBT sessions and a combination of prescription drugs. Sadly some people are just programmed to be pessimists. If this is really affecting you, then I suggest you go see a doctor. You will never be 100% happy all of the time, but if you are happy 0% of the time, then there is always room for improvement.