Hence the question: Has anyone got any tips on blocking out negative thoughts that are firmly rooted in past experience?[/quote]

As a last resort, alcohol may be an option. On a serious note, I took a page from the actor Jack Lemmon's book. Prior to doing an important scene, he would say to himself "it's magic time". Importantly, this set the stage for a mindset that was more easy-going and productive and released some of the tension.

Good luck!