Smoking is very harmful during pregnancy as you well know. It is important to start prenatal care early and when you do see your OB or a midwife , please talk to them about your smoking and how to quit. They will be more than happy to give you advise and literature on the subject. Many women have similar concern and many are able to quit their bad habits in order to have healthy children.
It is quite amazing how what we find impossible to do for our own sakes, we women are able to do for the sake of our children. It shows our strength and the strength of our love for them. I know without a doubt that you will be able to do it as well. Good luck to you.