As human beings, we have a tendency to be biased towards the negative. Some of us tend to focus on the negative side of things and dwell on them causing unnecessary stress and anxiety. This is what experts refer to as the "Negativity Bias", when the negative events have a greater impact on our brains than the positive ones. This may be one of the reasons why we find it difficult to overcome bad experiences or for traumatic events to linger in our minds.
How then do we overcome the "negativity bias"? I think self awareness is an important step that can help us manage these negative thoughts. Try to focus more on the positive things in your life, make it a daily habit to think of good things or whatever it is that makes you happy.
Remember that we cannot stop negative things from happening, but we can choose how we deal with them.