I'm stuck in a toxic relationship, so I've decided I'm not going to smoke.
Why would you stop smoking for the sake of a toxic relationship?
I'm going to cut all the fun out of my life.
I see you seem to be going through a hard time right now. Being in a toxic relationship wont help you though, this will only make things for worse for you in the long run.
I'm going to be fine. I just want to show that I can take it without smoking.
What does it prove if you show you can "take" a toxic relationship? Quitting smoking is hard enough on its own. Letting yourself be hurt by a toxic relationship doesn't make you stronger just because you don't smoke; it'll hurt you more.