Is smoking cigarettes a sin and if so why is it so hard for me to quit?
Smoking is an addiction (as proven by your inability to quit). As you also know, it is a habit that is harmful to the body God has given you and in which His spirit dwells. “For ye are bought with a price: therefore glorify God in your body, and in your spirit, which are God’s” (1Corinthians 6:20). Certainly, we cannot glorify God with cigarettes. Paul also states, “all things are lawful for me, but I will not be brought under the power of any” (1Corinthians 6:12). He means that though there is no law that says he cannot do something, there can still be reasons that it is wrong and one of those reasons is when a habit brings us under its power. That is, if I cannot quit, it becomes an addiction and that addiction is a sin. Clearly, smoking is a sin.
That does not mean that it is easy to quit and, although I have never smoked, I know how hard bad habits can be to quit. Since the addiction is also a chemical addiction to some degree, this makes it even harder to quit. Yet, God has all power and is able to give us the victory over such habits. I grew up with my dad smoking a couple of packs of cigarettes a day. I remember when he went to the altar one evening at the end of the church service. I found out later that he had given up his smoking. I remember him gaining some weight at that point, but to my knowledge he never smoked another cigarette. You must remember that God is able.
You must keep taking it to the Lord. When you fail, look at the circumstances of your failure and see what else needs to be changed about your habits and heart. God may use your struggles as an important training ground for you life in a broader way. Just remember, He is able and willing. We fail because of lack of trust in Him or lack of obedience to Him, not because He is lacking in any way. May the Lord give you full victory in your quest to serve Him.