Often the individual realizing they are not alone internally says it feels as if she is being sucked into a black hole or falling into an endless pit. The good news is that DID is highly treatable.
Regardless of age, when someone with DID begins to become aware of their dissociation, the typical response is fear and confusion. Noticing the gaps in time; having friends tell you that you behaved "totally out of character" in a recent situation; people claiming they know you from a party or business function and having no memory of the event. The most frightening, in my opinion, are fugue states. I had clients who routinely found themselves in their car miles from home in an unidentified parking lot. Or "someone" went shopping and the person has no memory of the drive or any part of the shopping spree. It's crazy making stuff.
Then what happens? Hopefully the person will seek a therapist experienced in treating DID. That is not so easy. Far too few doctors and mental health professionals acknowledge DID, possibly because it's too complicated and time-consuming to understand, let alone treat. Add to that the propaganda that multiples are fakers and therapists worldwide implant similar memories of childhood abuse into their clients. I would ask, "to what end?" WHY would an ethical therapist even want to implant a memory? If the client is there, she already has issues. I wish someone would answer that question.
If you are a survivor seeking a therapist, a good resource is the ISST-D website where you can look up a therapist by state. Another strategy is to call a number of therapists in your area and ask if they can refer you to someone who treats dissociation. The therapist with the most recommendations is a good start.
