Tough situation.
Some ideas...
1. Try and get her help. She's obviously getting high selfishly, in spite of how you feel about it. Can you identify a way in which you can either bring help to her or get her to help? Call rehab, or drag her into the car if you have to. Or trick her, and take her there yourself.
2. Substitute something for the pot. Does she like to read? Buy her a book, but she has to stop smoking for a day, 2 days, then a week, etc...if she wants to keep on getting books (or whatever else) from you. Does she like to eat? Promise to buy or cook her something, only if she wont light up. Take her to the movies under the same circumstances? Play cards with her? Go for walks? Think: things that are worthwhile to her.
3. There could be more, and no doubt others will mention more. But the last Ill say for now is there comes a point where you have to look out for yourself. You might do better focusing your attention away from her, at least for now, and towards what YOU want to do in life.
My parents didnt drink, smoke, gamble, anything. They bored me to death, instead. But its the same deal, I looked elsewhere to keep me from going crazy. That 'growing up' thing we all do..
Good luck..