Great question. I agree with Dean and Rangy and want to add to it. Youcandoit is exactly right...understanding shit like this will help everyone avoid mistakes.
Shutting the door is the key. I made nicotine part of my life. I made myself an addict. Addicts are excellent liars...very similar to con artists. The first person a conman fools is himself. I can never escape addiction because I made it part of me. I made it a part of things that I don't want to forget...baseball, camp, college, hunting trips, good times with friends, etc. I added nicotine to everything in my life from age 15 to 40...like mixing in a big fucking turd in my ice cream and trying to eat around it.
I conned myself for all those years this way. I could rationalize anything. I'd set future quit dates, lie to my family (parents first, then wife and kids more recently)...real shithead stuff, you know? I always knew it was bad for me, that I had to quit, that it would kill me, etc. But I had made it part of my life...part of me. Now, my challenge is ignore the nic bitch and focus on the part of me that I do like...or eat around the turd, so to speak.
The key for me was finding my motivation to keep the door shut. I had already lied to everyone and everything in my life. What was going to be different this time? One thing...I was quitting for the part of me that I like, that I'm proud of. I can keep the door shut because it makes me a better person...better for my wife and kids, better son to my parents, a better brother, a better uncle for my nieces and nephews, a better example for my kids' and their friends...a better me in every fucking measureable way.
I have fought for 586 days. Tomorrow, I will fight again. It has become easier and easier with each day as I put more ice cream in my bowl. I just have to remember that the turd is always in there, too.