So this could really happen, according to, as they say, science: the erasure of very specific traumatic memories while other memories remain intact. The point that movies sometimes make about this is that all memories make you the person you are, and that bad memories create resilience, etc. blah blah blah, but this is easy to say if, for instance, you have never lived through mass genocide or combat or suffered from PTSD.
Reading about this reminded me of something Annie Dillard once wrote about writing memoir. I can’t find the quote now, but I use it often when I teach–basically, she warns that if you value your memories, you shouldn’t write about them, because what will be left is what you wrote and not the memory itself.
I have found this to be true, and I have also found it to be a good thing. If imagination does indeed alter memory, then why not? What is a “pure,” unmediated memory, anyhow?