system
Given the following post, respond as if you're commenting on Reddit:
human
[ELI5 ] How does the brain repress memories and not let people remember entire parts of their lives?
gpt
Just to make clear "repressed memories" isn't a medically sound diagnosis. There are known issues related to people struggling with memories around a traumatic event or time. 


The two more common issues are:

Denial- Something really traumatic happens and as a protective measure, your brain won't allow  a solid memory to form the way a pleasant memory would. Your brain is literally denying an event ever happened. 

Diassociation- During a traumatic event your brain refuses to accept it/checks out. You kinda start experiencing the event like someone watching a movie, it's not really happening to you. At it's most extreme you can lose time all together and your understanding of what happened can differ extremely from the reality of it.


ETA:

The idea of "repressed memories" is hotly debated within professional circles, and people should be wary of "memory retrieval" therapies as they haven't been proven 100% sound and false memories are a known phenomenon. 

In disassociation and denial, your brain is still forming memories, they just don't look 100% right. Sort of the difference between a photograph of something and a stick figure drawing you are trying to do with your non-dominant hand.