I'm upset that my significant other decided to start smoking.
Why does her smoking make you mad?
I think she's too young to start smoking.
I understand you being concerned about your significant other's health, but if she's old enough for you to date her, then she's old enough to make her own decisions. If she wants to smoke, then you should let her do that.
It makes me angry and I have the right to do something about it, like get mad or cut her off.
You have a right to be upset, but not to tell her what to do. If her smoking bothers you so much then you should stop seeing her, rather than trying to get her to change her behavior.