It's all in mind. Just say you don't want to do it anymore. Always find a way to convince yourself not to do it.
For example I have always been stubborn, so when I try to achieve something what I do? It's simple I talk with myself. For example let's say smoking is the problem. That part of myself tells me how it wants from me to continue smoking, poison my body, spend more money, and then usually other side turns on which says, I'm in control and no way I'm going to let some lame smoking dipshit win.
It may sound funny or dumb but trust me. I'm very disciplined guy. Since im into bodybuilding and fitness I have to train every day and eat specified food everyday. It's not easy and as time goes you learn to control your life and your actions. You see what's good for you and what isn't.
I understand that it's hard as it's a drug, but even tho it's a drug you can always achieve what you want only if you REALLY want it.
I would even go for extreme measures if that's what it takes, buy food for 15,20 days and try to go out as least as possible, don't come anywhere near shops, go straight home watch movies, do anything what will keep you busy and not let you think about it. Lock yourself in a room, anything. Like I said, if you really want to achieve your goal, you will only if you put in 100% effort.
Smith out.