Ok...um....yeah. She breaks up with him after one fight? She randomly charges back and forth between two guys for apparently no reason? And Haldir is acting pretty stupid if he just randomly accepts her back without saying "gee, maybe I'll get hurt again"? And why is Galadriel being so creepy to her own daughter?
Ok...well...why exactly would Nienna go insane if she found out that someone else loved her????? That kind of defies logic. And why would she go crazy and assume that haldir is lying and that he must not love her?? And where was there any indication, any reason, that she would go to Legolas over Haldir? I'm sorry...but this is all highly improbable and very strange.
Hi there -- I just had some comments about your fic so far. Well...your summary gives too much away, I think. You should give those details away throughout the story instead of right off the bat.
Also, this first chapter was a bit erratic. The characters emotions change extremely rapidly and their motivations are unclear (why exactly can Nienna not return until she has produced a family? Why can't Haldir return? Why is Galadriel so mean to Nienna -- surely it can't be because of Haldir only.) And when they are first walking through the woods, Nienna tells Haldir to "prove" he loves her by kissing her...despite the fact that they have already said "I Love you" before. It just makes her seem manipulative (and with the part that said "their lips crashed together" that was slightly strange vocabulary). Also...you don't really need tags at the end of the dialogue so much.
You have a great concept here, but I think you're dumbing it down a little too much (by focusing on cheap emotion like "crashing lips" or Arwen yelling after about a 10 second exchange for no reason really) without really explaining important things like how the heck she'd make it out of Lothlorien like that, for example. I don't mean to be harsh...just want to help :)