What motivates Diana (major spoilers!)
After reading many of the comments here & re-watching the movie I've come to the conclusion that the filmmakers (along with, apparently, the book's author) give Diana too many reasons to choose to die in place of Maureen & thus dilute some of their own themes, creating ambiguity where none ought to be.
A number of readers have commented about how dysfunctional Diana's projected adult life becomes, largely because of guilt at having survived while her best friend didn't--(unless of course the apparent dysfunction of her adult life owes more to its hypothetical nature than to any survivor guilt on Diana's part). Thus Diana chooses to die because her only alternative is a less than satisfactory life.
Okay.
But why include the quotes about conscience being the voice of God in man, or the heart being the strongest muscle; why recall how much she loved Maureen & how much Maureen loved her ... why emphasize those things if her decision is based simply no not wanting to survive at the cost of someone else?
After all, suppose she'd been caught in the bathroom with total stranger & had indicated to Michael that he kill the stranger ... if guilt is the issue would't her subsequent guilt be nearly as a crippling?
I think the issue is one of conscience & morality. Diana realizes how much she loves Maureen & one of the moral imperatives of love must be the willingness to sacrifice to ensure the well-being of those we love, even to the point of sacrificing our lives to save theirs.
That in my opinion is what motivates & validates Diana's decision to die in place of Maureen. That she also projects an unsatisfactory, dysfunctional survival in place of Maureen seems beside the point. Given her realization about conscience & her feelings about Maureen, it seems clear that her decision would be the same even had she projected a deliriously happy & productive life.
andy