I smoked weed for about four years from when I was sixteen until my sophmore year of college. I eventually stopped (I am now 27) because when I was stoned I'd get intensely paranoid and feel stupid. I would literally be scared to talk because I would be certain that anything I could possibly say would be dumb. That said, I recognize that this is a problem I had personally with the substance, and not a reaction EVERYONE has to it. At the end of the day, pot is just pot. The negative health effects are fairly negligible, by my knowledge no one has ever died from smoking too much pot and the same cannot be said for tobacco and alcohol. What's more, there isn't much of anything harmful about pot smokers. The worst that I can say about them is that if you're sober they can be kind of annoying to hang around, just like anyone who doesn't think that well. Also, they will occasionally come up with ideas that are just moronic. I remember one time in college, some friends of mine decided that it'd be a great idea to go outside and start fighting each other with big sticks they'd found in the woods. Probably not the best idea they'd ever had. On the whole, the effects of weed seem to be boring conversations, laughing about dumb crap, enjoying Family Guy more than someone really should, and a strange desire to build more ways to smoke pot. And eating. But boredom does a lot of that too.
The biggest advice I could give you, relax. This issue you have with weed, it seems to be largely unwarranted. If you can't come up with a good argument for why this particular substance is causing you such agita, you probably should stop harping about it. If you don't like hanging out with your girlfriend when she's stoned, don't. Frankly, I don't think you have the right to tell her you wish she wouldn't do it at all. You don't have good reasoning on your side, you have a gut feeling. That is rarely a good metric for making informed decisions.
I am not among those who thing you need to smoke pot. You don't. Some people enjoy it, some people don't. But you do have to get over this. It's not about you. Some people just like to get stoned.