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 consult your doctor as you will get the best support and guidance while going through this process. Wishing you the best of luck!