I don't see a big deal here. There is absolutely nothing wrong with pot on it's own and pot is the least destructive drug you can possibly do. If she isn't letting pot come in between your relationship, I don't see why you have to make such a big deal out of her pot use. If you really loved her you would accept this part about her. It's cool to tell her how you feel and try to come to some kind of compromise but judging her and forcing her to change seems to be a bit unfair to me. If you're that bothered by her pot use then chances are she isn't a good match for you.

Honestly, cigarettes, alcohol, and prescription drugs kill more people annually than all of the illegal drugs combined so I don't see anything wrong with letting her have her pot. If it makes her happy and she isn't hurting anybody else than who the hell cares? Let the poor girl have her pot.

Not to mention, pot has proven health benefits and is even listed on PC under the "alternative treatments" section as a rated 6/10 treatment method so perhaps maybe you should be a tad bit more open minded?