Replacing negative thinking with positive thinking does not work for me either. What does work is recognizing a negative thought when it arises and not resisting it ... gently let it go and come into the present moment because problems don't exist in the present, only in the past which involves thoughts of guilt or regret or the future, which involves fear and anxiety. We can do nothing about about the past or future and the present moment is all we really have anyway.

The work of Eckhart Tolle (wrote 'The Power of Now') has helped me with this. I am getting much better at it but it does take some awareness to see those pesky thoughts when they come.