We must let go of the bad things that happen to us, especially if they are not rewarding enough to remember them.
It is not an easy situation, since any bad thing that happens to us will mark our memory and we will always try to remember it, however we should not only focus on the thoughts, since there are circumstances under which we have to move away from some person or some place, so other people even if it hurts must let this happen.
The need to let go arises because many times even if it hurts us, it is the only way for people to progress, we can love another person very much, but if we know that letting go will progress, then that is what we must do. As far as the thoughts of the past are concerned, there are some thoughts that we must also let go, whether they give us gratitude or discomfort, but if they do not let us move forward, then we must let them go.
Let's suppose that we come from having survived a war, where in spite of the bad moments we have lived we have been able to survive, we lost the house that we loved so much, then if our thoughts are always remembering how much we liked it, the most probable thing is that we put the attention to those memories that will not make us concentrate in new ideas to have a new house.
Thoughts can be many, some come to our minds and stay, other thoughts come and go, but the important thing is to keep the memories that allow us to move forward, any thought that means a stumbling block for us must be let go.