While it is always good to look back in history to see where we have come from, it is more important to look forward to see where you are going. If we continue to look back we will never progress.
The same can be said for mental well-being. When you have suffered a significant negative event in your life that has caused distress, it is alright to focus on it for a short period to establish why it happened and to see if you can prevent it from happening again where possible. Then, move on.
Focus on the positive aspects of the event, what did you learn from it, then look to the future. Where do you want to be, what do you want to achieve, and how can you get there? Work towards achieving something that excites you. Focusing on the positive will help you to forget your negative event by replacing bad memories with good ones. The addition of excitement about the future will create endorphins and other good chemicals to restore your well-being.
When the negative event pops into your head, acknowledge then discard it and again look to the future.