You can do it - I quit smoking when I learned I was pregnant (last cigarette smoked while waiting for the pregnancy test!). When you feel stressed and want to smoke, drink a big glass of water and go for a walk. Or have a healthy snack - you need to gain weight anyway, right?
My best friend smoked through her pregnancy and ended up having placental abruption and delivering at 26 weeks. Although (thankfully) her daughter survived and is fine now, she was a real miracle. She weighed less than 2 lbs, couldn't breathe on her own and needed heart surgery. The doctors said that there was a very good chance this was because of her mom's smoking during pregnancy.
I'm sorry if this post sounds alarmist, but I am trying to scare you a little bit - you may feel invulnerable, but your little baby is not.
Good luck to you! You can do it!!!!