A relatively new revelation for me is that it's practically impossible to "fix" or replace negative thinking. It's like trying to bite your own teeth.
The only way to eradicate chronic negative thinking is by not thinking at all. Shut down the inner monologue completely with regular meditation. It works for me.
EDIT: I use these meditation posters to instill a positive message subconsciously while I clear my conscious mind: http://zenpusher.com
There is nothing wrong with negativity, you just have to, like everything-else, take it in moderation. The world both sucks and is awesome at the same time. We are constantly "rewiring" our brain, it isn't rework or wasted effort. There is no one-configuration for your brain. It is. It sits there in your skull and you carry it around and have it experience things.
in other words: brain is just like your muscles. If you train your arms you will have strong arms, if you train your legs, you will have strong legs.
If you train to find the downside in every situation, you will get very good at it. But you can also train to find positive aspects in every situation and you will get very good at it too.
The only way to eradicate chronic negative thinking is by not thinking at all. Shut down the inner monologue completely with regular meditation. It works for me.
EDIT: I use these meditation posters to instill a positive message subconsciously while I clear my conscious mind: http://zenpusher.com