NTA.

Logically, your position is 100% correct. At the same time, you're within your rights to determine what you consider tolerable habits in a partner, and yes, you're even within your rights to change your mind. Nevertheless, she's not obligated to stop for you. You can try to motivate her to stop, but that's an uphill battle and you can't make her stop. If she won't, you should leave, because it's not going to stop bothering you and you also should consider what the implications would be for your future children. You want them to pick up a smoking habit from their mother's poor example, or develop health conditions from secondhand smoke?