I think the first line of the movie should be "I do not get the girl in the end." And then you should lay out in typical rom-com fashion why they will of course get together in the end.
I think, at the end of the second act, when you reveal how the boy and the girl got together in the first place, the scene should go like this. At the end of the night at the local watering hole, our male love interest, who is the bartender, gets up to do karaoke and sings this song:
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By the end of the song, the whole bar is singing along. As the bar clears out, our female love interest offers our male love interest a few lines of her own stash, the white lines in the song being an obvious reference to cocaine. Under the neon lights of the parking lot, they have coked out sex in the bed of his pick up. It is through this scene we learn that our two lovers' entire relationship is based solely on mutual addiction, the subtext of which has been in all the previous scenes leading to this point.
I think in the third act, you could broaden your scope and apply the theme of addiction to the genre of the romantic comedy in general. The audiences' expectation that the these two will stay together because they are the protagonists is also the lovers'. You could use this opportunity to make a statement about how the audiences' addiction to the escapist tropes of the romantic comedy can be just a soul-polluting as cocaine, or something.
Oh, and starring Brad Pitt and Jennifer Lopez. Directed by Steven Soderberg.
