I am looking forward to seeing this film, if possible. What I got from the trailer is that she is a girl who has been over-confined for a long time by her illness (not her mother, as I saw it, who has been trying to help her) and she takes action on her own. A horse is a classic symbol of life, power, freedom, speed, grace, etc., not only on a physical level but a metaphysical one. I think she is reaching for life. The fact that it isn't going to work out on a practical level, I expect we'll see. Maybe the little toy horse represents in a humorous way the fact that her wish is a childlike fantasy. Still she longs for a whole life.
We have the example of Laura Hillenbrand who fell so disastrously ill when only a freshman in college. Her memory of riding horses as a teenager meant so much to her, she kept dreaming about it until she discovered the story of a forgotten racehorse, Seabiscuit and began her work of researching and writing his story. The fact that his potential and story had been as thoroughly unlikely as his achievement was high is a parallel to Laura's story too. Her aspiration, represented by a horse, gave her life purpose, meaning and eventually great success and respect as a writer.
When I can see the whole film, I might or might not feel Wild Horses was well done as a story, or an accurate portrayal of one girl with ME, but right now I want to honor the concept I see, which is the desire of a young person for a whole life, freedom, a healthy, capable body--whatever this horse means to her. But what the horse is to her is something real, tangible, and alive- what she wants is a real experience.
In the trailer she is even asked, repeatedly, what the horse signifies to her in a conceptual sense, as if this could clear up her "crazy" desire. But her response shows that what she wants is the real thing (the symbol not as idea but as a living experience).
I can relate to and honor this healthy desire, just as I believe it is not going to pan out for her on an ordinary level. We know this story is likely not headed for a Disney ending. Still her wish for life and willingness to act on it, whether rational or irrational, is something we can relate to and see value in.