Now, you can't either!!!
If I tell you, I'll start crying again. So many things that I can't unsee as of late. None of it good, all heartbreaking.....
Anything.
It depends on the impact it made on your psyche as to how long it stays in your short term memory or off loaded to long term storage.
There is some evidence that the transition from short term to long term memory involves a conversion from electrical "type" of memory to "chemical" type of memory. I think it was in a free publication I get from here: www.dana.org/ some years ago.)
While I can "unsee" some things that were terrifying or horrific, the "unseeing" just involves not having the original emotional reaction attached to the image; and in addition, it seldom crosses my mind again.
Just my personal experience with the concept of "unseeing."