Ok. Here are some suggestions or recommendations I'm going to make, they might not be relevant and you might of already tried them but they could be just some things to think about.
I could be wrong, but in most situations usually the parents are angry or overprotective because they believe they know what's best for you and know what makes you happy. This is usually a wrong assumption the parents have, but it does mean you might be able to appeal to that side of nature. Now, assuming you haven't done this, you should talk to your parents, in a nice calm and safe manner, and tell them how this is bothering you. Explain to them everything really, about your girlfriend, about your smoking, about what each means to you and how you can justify seeing your girlfriend. Then if your parents are even the slightest bit convinced, then maybe you can start to convince your girlfriend too.
I think, there might be a way out of this so you aren't at the losing end, I don't know the whole situation so I can't really make that judgement. But if you see the situation as more of a tactical approach, something you can DO, something that you can see as possible to figure out a way to solve it, you might just figure out a way to solve it. I hope things go well.