Smoking. I smoked from my mid teens til my early twenties, and then quit on the day I found out I was pregnant. It wasn't the quitting that was hard (having a tiny person growing inside is good incentive to stop!), it was the staying away from it after it was safe for me to smoke again. Even to this day, seven years later, whenever I am out at the pub with my mates it is sooooooo tempting especially when I see a friend make a roll-up. Thats how much of a hold that little nicotine bastard can have on you!