Hi Mellow,
You’re not alone. I have experienced this particularly towards the middle and especially end of my relationship, and I think it’s a combination of: a) our brains blocking it out because it’s too traumatic to handle at the time, and b) some of it not registering in our brains when it’s things that are particularly commonplace. For example, a lot of the verbal abuse my ex hurled at me is gone from my mind, simply because it happened so much that it was too commonplace to remember specific events.
I’ve been writing down things that I can remember before they too maybe disappear from my memory, but it may also be that as time goes on and we start to heal, things start coming back to us at points where our body and brain can handle it, because they sure as hell can’t when we’re in the midst of the relationship!
Sending hugs xx