To forget. Letting go of something is much easier, seeing as you still realize it is there, but don't pay much mind to it. As for simply forgetting, that is difficult, seeing as many things can cause you to remember.
It is easier to let go of something; just put it in the past. It's over and done with and nothing more needs to be done about it. This can be done by an effort of will. Forgetting is harder, and not really wise. If you forget an unfortunate incident in the past, you have learned nothing from it and are more likely to repeat it in the future.
I was dealt a shocking revelation a few years ago. It was so bad I became completely unable to function and had PTSD. Finally I had to drop it in Christs lap. Let Him deal with it. It creeps up in me in the quiet of the night, but I toss it to Christ again. I wouldn't be able to cope otherwise. He will take your burdens if you let Him.