The problem here may lie that your own vision of the story is already biased toward the metaphorical result you wish to obtain, but such perception might not be clear for the rest (this is something that every visionary struggle with). Just by simply looking at the script, if you ask me, it looks like you're trying to explain the dangers of schizophrenia rather than whatever symbolism you might be trying to cast there.
A story doesn't really need a pay off, but just like your teacher, I'm struggling to find one here. Fourty five seconds are more than enough, but maybe the perspective of a "car accident" doesn't really fit it. Why not simply have Lilly whisper to Miles ear "time to let it go" and have her close his eyes? Why the need to show an accident at all? I've always valued more being suggestive over being graphical, and maybe trying to focus more on that "lilly getting overrun'd while Miles hold his head in pain" doesn't make the cut.
