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What is usually helpful in situations like this is not to try and block the irrational thoughts but rather to engage in re framing your internal dialog to think about something else (does not have to be related to the incident)
There are self help tools such as hypnosis CD, autogenic training, bibliotherapy or working with a counselor specializing in CBT/cognitive behavior therapy or a hypnotherapist specializing in NLP/neurolinguistic programming.
When you start thinking about the incident, immediately shift the thoughts to something else. Do not attempt to force yourself to halt thinking about it, simply observe the thoughts, accept the fact that you've allowed yourself to think about it and then focus on something else whether thought or action/doing something. You can start reprograming your brain (attached is the info on autogenic training you can practice yourself)
Change Your Thoughts Meditation CD: Do the Tao Now! by Wayne W. Dyer (Audio CD - Nov 20, 2007) - Audiobook