Today I've got Sharon's character Layla on the couch. Layla is a 30-year-old resident in a women's fiction book. She's got horrible scars on her body, but absolutely no recollection how she got them. Her parents died when she was 22 and already out on her own, but eventually Layla was homeless and forced to beg to survive. An older woman Roberta took her in, saving her, and she says Layla just doesn't want to remember what happened to her because it was so bad.
Sharon wants to know: Will Layla's memory come back of its own volition, or must there be a memory
trigger to make her remember. She's had this selective amnesia for several
years. Is that unusual?
When the brain experiences a traumatic event, it can do several things as protective measures. What you're describing isn't selective amnesia (which has the connotation that it's convenient for you to have forgotten bits and pieces of your past), but it's more like a repressed memory.
The main feature of this amnesia is an “inability
to recall important personal information, usually of a traumatic or
stressful nature, that is too extensive to be explained by normal
forgetfulness” (DSM IV-TR, p. 520). This is reversible, which is important if your author plans on you remembering your past by the end of the book. Dissociative amnesia is most likely reported as a gap or series of gaps in recall for aspects of an individual’s life history, and you've got a big gap.
In essence, your brain dissociated (split) for the time period of your tragic past. And yes, whatever it is you're not remember is likely horrific. The scars on your body is a major clue to the traumatic nature of what you're forgetting. And your brain has set up these defense mechanisms to prevent you from remembering--as a form of cognitive protection, if you will.
As for remembering, the sky really is the limit. I've gotten this question a few times, and there just is no formulaic order for how people remember, if they remember. You might remember snatches of the same memory over and over. Something you smell/hear/see/taste/feel might open the floodgates of your mind. Sky really is the limit.
From a reader's standpoint, we're going to want to know why you've got those scars, so for our sake (and reading satisfaction), I hope you do remember.
Best of luck to you. If you want to go deeper, and talk scenarios about how you might remember or other details, you know where to find me. It's only $14.99, cheapest deal for therapy in the world. :)