I'm going to guess that at some point in her life self-harming by cutting was a coping skill (not an ideal one, of course, but one that fulfilled certain functions for her). It was likely painful/ potentially harmful to the people around her who cared about her, and there were health risks to her associated with it. She has since recognized the harm it caused in her life, the way relying on self-harm to cope created other problems, etc. and that it arose because of mental health concerns, which is what led to her engaging in whatever behavior she engaged in that helped her stop. I'm also guessing this was a hard thing for her to recognize and a hard thing for her to stop doing (likely she also had to find new ways of coping that worked better for her). The crucial point, is that something tipped in her life that made her recognize that stopping was a healthy decision for her and one she wanted to make.

Stopping smoking is also a healthy decision for you. Smoking is also likely something you rely on because it serves a function in your life. It also disrupts your life (current relationship problems) and risks your long term health and the health of the people you chose to smoke around. While it is not the same thing as cutting behavior and I personally wouldn't compare the two if she hadn't made the comparison there are some parallels. One could even argue that if you regularly smoke around her your behavior has even more of a direct impact on her and her physical health (I'm not discounting the emotional health impact of being with someone who is cutting when I say this). Like her cutting, smoking also serves a short-term coping function (calming you down) at the cost of other immediate concerns and long term concerns and addiction is also a mental health concern.

The difference here is that she chose to stop self-harming because she wanted to do that. She made the decision and put in the work. You are saying that the only real reason you want to quit is for her, not because you actually want to. Unless you want to quit for you that is your decision. Just like she would have the right to continue self-harming if she chose to do so. You could decide not to be with someone who doesn't have cutting behaviors under control and she could decide not to be with someone who is addicted to nicotine. Those are both reasonable choices. She can also set a boundary that you don't smoke around her, just as you can set boundaries that she doesn't self-harm around you or in a home you live in with her. But you get to decide for yourself if you want to quit or not. You don't get to tell her that your choice to smoke is more or less bad than her prior choice to self-harm. While people will have all sorts of opinions on which is worse the bottom line is that they are both unhealthy ways of coping that are likely to cause more problems than solutions and that the long term consequences of both may be highly detrimental (or may not be depending on context and luck).

Stop arguing about whether your choice to smoke is bad, unhealthy, etc. It is. But you are still choosing to do it and lack the motivation to change and she either needs to accept that or move on. Also stop comparing your smoking to her cutting, there are differences and similarities. Listen instead to the subtext of what she is telling you. She is your partner and she sees you engaging in unhealthy ways of coping, has concerns and wants you to stop. She is comparing your way with hers because that is the language she has to tell you that she sees you hurting yourself and is frustrated that you don't really want to change. Address that piece and leave the tit for tat business out of it.