Fro you are right on.
But lets back up a little bit and talk about the old couple that fly with her from her home town to LAX. They represent her dreams, aspirations, desires, longing and need to be a successful actress.
The deal with the monster is that it is a metaphor for her inner self (for lack of a better term) the monster deep in her/our subconscious. Once she pays for the hit she realizes that the "monster within her" lets go of all of her dreams, aspirations, desires to be a successful actress. At the end it isnt the old people that drive her to kill herself obviously, it is her failure to acheive those dreams, aspirations, desires that drive her to kill herself. She knows that when she hands the money over for the hit all of her dreams go out the door and she has not choice but to kill herslef. If you notice the hitman visually hands her a key once she pays him. The key opens the box that the monster has where little old people come out. The hitman does not physically hand her a key to release her desires but does so metaphorically because he accepts the hit and her career/life are over.
As far as the cowboy goes, he is the reasoning that keeps her whole dream sequence together. He tells the director that he HAS to hire a certain actress but never gives a valid reason. At the dinner party when she snaps to and realizes that there is an actual reason why the director wanted the other actress and that he really didnt HAVE to choose another actress
the cowboy walks out in the background and her dream world comes to an end. I hope that makes sense. I'll try to answer better if you are a little more specific...