I smoked during two out of four of my pregnancies...i know it isnt an excuse but i had a lot of stress with my DP while i was pregnant and found it very hard to give up...
When i did smoke i was only on about 5 a day though...
When i gave up with the others i just found the best way was to go cold turkey and just remember my baby,and look at u/s pics or something when i felt i wanted one..
It wasnt easy at all though...
I hope you get on ok!