You're probably not going to like this answer, but you don't forget. As cliche as it sounds, the pain WILL lessen over time when the event is months, years, decades old. That's something you truly realize the older you get; it's weird. I won't even say the pain is going to go away, because some of my memories are still painful, but that pain is a fraction of what it used to be and fleeting.
Therapy can definitely help, but for extremely severe events, it's fine if you allow your brain to repress your feelings over time (though I doubt anyone in psychiatrics would agree, it is a means of survival. It's better to repress than end up dead). If you allow life to move on it will become less painful, but if the event has brought your life to a stand still, it may be wise to seek professional help.