As a human, God already said that we are the kind of creators who got the brain. It is not just a normal brain but it comes with mind. It has the ability to think which is absence in animals brain. However, the mind that we have have a very high tendency to forget. Yet, we can remember a lot of things for instance facts, theories, memories or whatever it is but it is not as supreme as the computer hard disk that can store data for a very long time.
Why does God create a brain with a tendency to forget? From my own perspective which might be not as mature as you, I think that we really do have to forget somethings. There is no reason of keeping anything that can make self-destruction of yourself. Just let it go. There are still a lot of options that we can pick to get through this life. Just ignore any negative thoughts from haters towards you. They are not perfect. None of them are perfect.
What do we have to do to maintain ourselves is to place ourselves in a reminded condition. This world is just not for you. Billions of people suffered heavier than yours. B grateful then. goodnight.