Having trouble dealing with my girlfriend smoking. Any advice?

I don't smoke through personal choice for a lot of reasons and in the case of a lot of my smoker friends, I'm happy to peacefully coexist. I can be around them smoking without any tension or arguments brewing easily. However, my current girlfriend is the first girl I've been with who smokes and I'm having varying levels of difficulty handling it.

She got into it during a weird phase she was in a few years ago where her old friends were trying to do the whole antisocial youth thing, and peer pressure got her. She's still friends with all these kids and while she's out of that phase, all her family on one side smokes as well so she doesn't have a lot of positive influences or people supporting her stopping.

In my mind it is just another form of addiction and addiction fucks lives up, so when my girlfriend has an addiction it hurts to watch, you know? A lot of smokers say they want to stop with no real intention, but she's assured me in the frequent talks we've had about it that she does want to stop so I feel like it is something she actually plans on doing which means that I'm actually invested in helping her quit, unlike with my friends with whom I'm happy to leave to their own decisions. For the most part I've given her positive support, we've talked about strategies she can take and why it's best to stop now (three years in) rather than down the line because of all the negative effects of it, etc.

But here's the thing. Often it's me suggesting and her agreeing, but I can't see her actually making these efforts to stop. In fairness she's just come back from holiday where she hasn't smoked at all for over two weeks, which is great. However, she's smoked every day in the three days that have passed since then, and while she's been on holiday and during each of those days we've talked about her missing days out to help her get off it (she doesn't smoke every single day anyway, but it's the majority of days in the week that she does smoke). In those conversations she appears responsive to it but when she's around her friends (most of which who smoke) she always ends up doing it again.

I thought to post here because it's less a relationship question than a question of how I should best manage my feelings when a close one is doing something I just can't agree with. While I want what's best for my girlfriend and for her to achieve her goals, I know people simply aren't as passionate about not smoking as I am and there's a probability she'll never kick it, and maybe that she doesn't really want to. My problem is I'm invested in her stopping and when she continues to smoke, it affects my mood and causes tension between us. Most of our conversations about it are positive, focusing on how capable she is of stopping etc. but sometimes they become more of an argument, the worst being where I was so exhausted with my positive support and efforts being agreed with then ignored that I said out of anger that she's never going to stop (which I feel guilty about because I basically said to her I don't believe she can do it). Her reaction to me saying that was the most convincing I've ever heard her talk about wanting to quit which is what leads me to believe she's serious, but when she's socialising with our friends and heated relationship conversations aren't happening, she just seems to lose that commitment.

Does anybody have any advice as to how to handle my emotional reactions to her smoking? All other aspects of our relationship are great, it's only this that causes tension. Sometimes when she smokes it doesn't affect me and during the positive conversations I feel a lot of trust and belief in her, whereas other times (usually when I know it's been a few consecutive days of smoking for her) seeing her do it really affects my mood. Any help is appreciated.