Welcome to theLoveLogic. I am a neat freak myself so maybe I am not the one who should be giving advice to you on this one. I used to be a very messy kid until as I got older, I couldn't stand clutter. The way I see it is simple. If you asked nicely and she refuses to even make an effort, she doesn't care about how you feel. The main thing to remember is NOT to leave her due to being cluttered but out of the fact she doesn't value the same things you do. If you take great pride in keeping things nice and tidy and she doesn't...nothing is going to change her mind but HER.
Take care...we have some of the best members out there to give advice. I am sure you will have a second opinion on this one.