MySpace Isn’t Just A Website

I recently moved into roommate situation in a New York City apartment. I pay half the rent, yet my roommate is failing to give me half the space. There are three closets, all of which are filled with her belongings, and I have maybe 25% of the shelf space in the kitchen, fridge, and bathroom medicine cabinet to call my own. She is a pack rat and refuses to throw anything away to make room for me. What should I do?