Elisa has recently left home permanently, so the pretense of childhood is over, as the realization hits her that she is 21 and on her own. Losing her cell phone breaks the final bond of contact with home, and since she cannot speak to her mother on the phone, she realizes finally that she is really on her own, and that lifes circumstances will force this reality on her from then on , and that she has to start functioning as a responsible adult.
But the problem is that there are two people living inside of Elisa. The little girl is going to have to go. Maybe Elisa has to choose which one of them will live and which will die.
In the end she follows the script of the movie. Elisa will have to die (21 year old Elisa) so that the little girl (six year old Elisa) can live. Maybe little alter Elisa, in her dissociative state, and traumatized by the memory of the film, comes up with a plan to drown 21 year old Elisa.
Little Elisa is rejecting adulthood (and rejecting mortality), by killing off the adult 'alter'.
MPD Elisa may have seen her own adult sensual needs and sexuality as a nagging reminder of adulthood and mortality, as she shifted back and forth between alters.
This would explain why 'little Elisa' may well have scrawled the insulting epitaph on 'Big Elisa's' water tank grave, Latin Harry Potter style