Every writer of fiction, as a new story begins to take shape, has to choose whether to write it in first person or third. (Tom Robbins once wrote an entire novel in second person, but that was a stunt. No need to consider second person as an option.) Young writers are often advised to use third person. First person gives a narrative more immediacy, but it’s harder to control. With third person you have more latitude.
The immediacy is, I think, the point of writing in first person. “I gazed out at the river” draws us into the scene a bit more concretely than “He gazed out at the river.”
Recently on one of the Facebook writers’ groups, someone posted a paragraph and asked for comment. The paragraph described (or at least hinted at) a rape scene, and the narrator, who is evidently being raped, says at some point “I don’t remember” some of the details of the scene. She has blocked them from her memory.
I’m not an expert on sexual trauma. I’m sure people do sometimes block out painful memories. (Other times they may wish they could!) Nonetheless, I maintain that this is not the right way to write a first-person narrative, except in special circumstances. The advantage of first person, to reiterate, is its immediacy. First-person narration drops the reader directly into the scene. However, saying, “I don’t remember” pushes the reader away. It opens a yawning gulf between the reader and the scene.
If the first-person narration is in the form of a letter or series of letters being written by one person and addressed to another (an epistolary novel) the situation is quite different. This is after-the-fact narration. Or if there’s a frame-story in which we see the narrator describing past events to a psychologist or a trusted friend, again it’s after-the-fact narration. In this case, the narrator can easily say “I don’t remember,” either truthfully or in order to avoid explaining something to the presumed (fictional) recipient of the narration.
If your story is not in the form of after-the-fact first-person narration, then your first-person narrator must reveal the details of the events as they occur. (We will make an exception in the case of Agatha Christie’s The Murder of Roger Ackroyd, in which the point of the first-person narration is specifically to mislead the reader.)
In the case of a painful scene, be it rape or sudden death, the first-person narrator need not be explicit about the penetration, the spurting blood, or whatever. Circumlocutions that spare the sensibilities of readers can certainly be employed, and every good writer knows how to use them. But “I don’t remember” is cheating. It’s bad writing, because it shreds the whole reason why you chose first person to begin with. Don’t do it.