Nicotine, one of the most addictive substances you will ever know. Nicotine is the reason you continued to smoke after trying it a few times; nicotine is the reason why you have had trouble quitting in the past; and nicotine is the reason why you feel so poorly during the first days or weeks after you have quit.
definitely better to quit smoking cold turkey, the sooner you completely quit, the better it is for your overall health.