It's stubbornness and her inability to listen that is fueling this current decline. At the same time, you could be contributing with how you approach the subject and what you are upset about with her. It could make all the difference in who has more responsibility in this, although you both still do.
Ending the argument without a solution and pretending like it didn't happen is not an effective way to solve things. She might not like hearing it and will probably think "I don't need to change for him, he should like me for me" kind of attitude. Threatening to end it is just a way to pacify you until you stop bringing it up. It's low blow tactics and that's not cool.
But like I said it depends on what you are criticizing about and how you are approaching it too. If you point blank accuse her of doing certain things or being a certain way, it probably won't get you what you want. So you have to try different approaches, maybe suggest solutions instead of problems, etc. etc.
Waking up next to a beautiful girl,
Step outside and say hello to my beautiful world.