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