Anybody who has seen the movie Labyrinth very well remembers the Sarah, a girl who believes in a world where Goblins are inhabitants and they have a mean Jareth the Goblin King. Sarah believes that there is indeed a magical world besides the world she is living in and she even visits it and comes back home and in overall process making a lots of friends who visits her often then, Now i have a simple theory, that how Sarah slowly gets turned in to Marion Silver, yes i am talking about Requiem for a dream which i think depicts the future of Sarah where she is all grown up now and is searching for the answers and peace.
According to me, what actually happened is Sarah in the end asks for all of them to come forward and live with her and all the mythical creatures and goblins starts visiting her, but eventually as she grows up, they all starts disappearing, first she thinks its just normal they will come back, but gradually the visiting stops.
Sarah finds it disturbing and highly upsetting and she starts missing them so much, she calls for them while reading the very book she reads in Labyrinth. She also searches for all over the place. Finally she starts thinking that it was all her imagination and to give that a boost she starts taking drugs, but even though she takes the drugs and imagine the goblins and the creatures besides her, she vaguely recounts the real feel of them having actually beside her and having a good time.
Slowly the drug addiction takes over and the sweet Sarah fades away in to the personality of Marion Silver who is still trying to get back to her mythical friends through drugs and searching for a way to be happy again.And that is how people Marion Silver is actually Sarah. Loosing her mind.