I find it so hard to stop smoking. Am I weak?

No. It doesn't make you weak; it makes you human. Cigarettes are so addictive that 70% of smokers say they would like to quit, yet they still smoke.
Most ex-smokers try to stop a few times before they manage to quit for good. It's the nicotine in cigarettes that's physically addictive. It reaches the blood stream within a few drags, and alters various brain chemicals that change mood and concentration, which smokers find enjoyable.
Stopping smoking can lead to intense feelings of anxiety, irritability and depression, which smokers crave cigarettes to get rid of. Using nicotine replacement therapy (NRT) can help people break this cycle.