A Key Point To Understand It - Spoiler - Read After 1st Viewing

"The Life Before Her Eyes" is all about point of view, everything is being seen from Diana's POV except for the rest room scene which is from the audience point of view (where Diana is communicating non-verbally with viewers). She is simultaneously flashing back to events in her friendship with Maureen and to parallel events in her imagined future.

Essential to understanding this process (and the film) is to recognize that she and Maureen have already spent a lot of time together imagining each others' futures. And that these times are central to Diana's thoughts and decision-making process in the rest room.

I think that the key sequence (which I only really picked up on during a later viewing) is when they are walking together on the sidewalk as the lawn sprinkler showers them with mist. The sequence is repeated later for emphasis. During this Maureen talks about how she used to watch flowers in a heavy rain. That the flowers get crushed by the rain, yet amazingly "some" of them are able to recover and bloom again as if nothing had happened. Diana flashes back to this and it is central to her decision. She believes that if she is killed instead of Maureen, that it will crush Maureen but that she will recover and bloom again. She contrasts this to the life she is imagining for herself if she allows Maureen to be killed, and the result is reflected in the crushed, decayed, and withered flowers that are symbolically shown in the later scenes of her imagined future.

Diana has also come to believe that dreaming is the most important thing in life, and in the world she imagines for herself shutting out the nightmares means losing the ability to dream.