I started at 16 and I'm 26 now, so you have 2 extra years on me, but the way I stopped was a combination of some of the things mentioned above. A little willpower, a little attitude change, a little "just not lighting a cig", and a little of it being "hard because of addiction".
It helps if you make another fairly large change in your life at around the same time, such as moving house or changing job, or taking up a new goal in life that you intend to work towards, on top of your attitude change, because you're more likely to feel like you're no longer in the same situation as the person who used to smoke all the time. You are a person with a somewhat different life. That is a cliche, of course, but a significant change in your life does have a big psychological effect in making your routine less rigid and more pliable, your little bit of willpower goes a lot further in that situation, and your little bit of attitude change is also strengthened by the real tangible change in situation, making it much easier to stick to.
Every time since then, that I've been tempted to smoke, I remembered how much I had been coughing and clearing my throat, and how ANNOYING it was, and how much I didn't want to be doing it in 5-10 years time, at an even worse level, with little kids running around complaining to their mother that it was annoying them too. Even just the thought of that is so much more annoying than the annoying feeling of wanting to smoke, so if that little joke of a feeling thinks it's going to compete, all I can do is laugh at it, because it's not.