It could be dementia, but the timing seems too soon to be dementia, as it can take years for dementia to get to the point where you get so forgetful that you end up down the deepend, not to mention dementia also tends to be more common in older people, so it's unlikely someone in there 30s would have such a severe case of it. Not to mention that Susan wouldn't have be able to keep even remotely accurate time in her journal, as if it truly was a later stage of dementia her sense of time probably would have been non-existent. It definitely could be someone experiencing the early stages of dementia, along with perhaps dissociative identity disorder, but I don't think it was dementia that truly killed her.