Probably the most difficult part of quitting smoking is the feeling that you're being denied something you want and feel you have a right to HAVE. I would assume this would be true of anything...if someone said you could never again have your favorite food, drink or other item of comfort it would be the same thing. Nicotine IS an addicting drug and your body does go through a period of withdrawal when you quit smoking but that doesn't end up being the part that's hardest to overcome; the part that is most difficult is breaking a habit you enjoy and are used to.
Perhaps you smoked while you talked on the phone or while having a drink or with friends? Maybe, like me, you smoked while you watched movies in your apartment. Maybe you liked to smoke at the end of a long, stressful day, right before you went to bed or right after you woke up. Possibly you just liked knowing every 15 minutes or so you were going to get a "fix."
When you quit smoking you have to re-train your brain, to teach yourself how to exist without the periodic "reward" that a smoke break is for most smokers.
Most people who don't succeed at quitting smoking have approached it from the angle of being something they have to do that they don't WANT to do. They see it as a form of punishment and are thus not as likely to succeed.
The trick is going into it all with a game plan and a good attitude. Here are some suggestions that helped me when I quit several years ago; I hope they can help someone else too!