I guess it's human nature that we should tend to dwell on those things which went wrong, often failing to recall those which were successful, happy and exciting. Sometimes it seems as though we believe that constantly replaying the sadder, badder stuff might make it all go away, whereas it often just becomes increasingly visible, usually far more than its significance deserves.
If you're anything like me, you've probably got good at chewing over negative events, seeing the pictures in your mind just as clearly as they were when whatever it was first happened. Often, however, a kind of distorting filter gets placed in front of the lens, so the pictures you see are a misinterpretation of what really took place. It may be difficult, but there's a lot to be said for thinking like a detective when these nasties show up in your head. Was it truly as awful as you apparently recall? Was everything about it as bleak as you remember? And most important of all, is there honestly any value in playing it back as often as you do?
So here's a thought. If you've become an expert at creating these remembered images, why not put your skills to better use now and again by re-running memories of an especially good and happy event instead? Visualise the sights and sounds, and concentrate hard on re-imagining thoughts you could have experienced at the time. You may be surprised how well this can work, and how swiftly it could bring a small smile to your lips.