Oh man, i have the exact same problem. Whenever i see something that my mind perceives as scary, it WILL haunt me before i sleep. Like you, i also have a rather good memory, and an extremely vivid imagination as well, so it can be troubling at times. I find the best solution being just to let your mind drift. If that involves going through more scary thoughts, then so be it, because eventually your thoughts will go elsewhere. In essence, what William said.

Or, if that fails, watch/think of porn. The scary thoughts will easily be shoved aside by the power of your RAGING BONER. (and then hope you don't have an all too wet dream)