I turned 26 when I heard myself coughing like a 66 year old chainsmoker with cancerous lungs, found I was unable to run up stairs and out of breath after carrying groceries inside. I had to have a cig every morning so I would be able to have a shit at all, but if I did... that first drag sent me rushing to the bathroom, it got so bad, I had to light the first one while sitting on the loo, or i'd shit my pants.
That's when I found myself disgusted with myself. I stopped, I simply stopped. From 38 cigarettes per day to 0. I am so happy it worked, because I am a very easily tempted personality and tend towards addiction in anything that gives my brain pleasure.
It took a year before I completely stopped coughing and two years before I could run up those stairs again, but one day I simply realized "Oh my! I'm not out of breath. What... what happened? Oh, yeah I quit smoking! Damn this feels nice!"