How to get someone to stop smoking weed?

[Aaron]

My girlfriend lives a few states away from me and we don't really have the means to see each other all the time (it's an eight hour drive and $300 flight). Two weeks ago, she told me that if I got uncomfortable with her using it, she'd stop. Last week, I confronted her about her weed use because I was very uncomfortable with it and she got really mad about it (I was ok with it before, but I thought about it alittle more thoroughly and changed my mind). She then told me that I was buying into the bullshit that the government, school, and media feed us. I left her alone for a few hours and she texted me, telling me that she wasn't going to stop because it relieved her stress and "I don't see the point" wasn't a good enough reason for her to stop. Recently, I've been doing some research of my own and I want to confront her about it again, but I don't know how to go about it. We both want this relationship to last, but her social use of weed and my jealousy don't mix too well. She also lives in a state where weed has been decriminalized. Please help.