I don't want to let my girlfriend smoke or drink.
What makes you not want her to smoke and drink?
There is something related to that, she has been smoking since high-school and now she's addicted to it as well as became a heavy drinker as well as a stoner.
If she has a serious smoking and drinking problem, the first task is to get her to realize herself that she has a problem. When she does that, then you can move forward and address the problem with her doctor who will be able to provide expert advice.
She is already a grown up and she's smoking due to stress.
Maybe you should try to talk to her about what is stressing her out. She might smoke less if she has someone to talk to about what is going on.