This article: Positive Thinking Tips: 10 Ways To Overcome Negativity And Become More Optimistic has some good ideas on how to keep a positive attitude. We want to be careful, however, to realize that just changing thinking is not enough.
It turns out that negative beliefs are stored as feeling states in the body/mind prior to thought. The initial intention should be to resolve and integrate the feelings first and after that lighter thoughts, or better feelings about the self will follow.
Doing EMDR on the more deeply embedded trauma is essential, a body movement practice such as a yoga is important, as is adding in great nourishment including omega-3s, protein, and live foods at each meal.
Stay with the ‘felt-sense’ of all experiences – if you’re working with thought only, then you’ll revert to negative thoughts and believes when under stress.