I believe the reason that we can't seem to remember the deepest details from the breakthroughs and the reason the memories we do bring back seem to fade is that we are not experienceing these events with our physical bodies. Since we are experiencing these events with our "spirit" if you will, the memories are not recorded into our brains. As we are anchored back into our bodies, and we begin to experience the physical world with our physical bodies again, perhaps we simple let go of the "spirit" memories. Or perhaps when we journey outside our physical bodies again, the "spirit" memories are there waiting for us. This would explain the feelings of "This realm, event, Entity, etc., seemed familiar to me" that some people report.