Anxiety may be the number 1 reason why people don't stop feeding their addiction to nicotine. Here's an ideal drug for making someone rich: market a drug that doesn't cause you to be high or interfere with your cognitive function until you stop using it, and then cause extreme anxiety or even panic attacks. Anxiety is what caused me to cave in every attempt to stop in the past 3 years. Here at KTC you have a bunch of bad ass quitters who stay quit every day because they are willing to face the anxiety, tolerate it, and find better ways to deal with it than take a drug. They face their anxiety without bawling like an infant back to dip, I can do the same.
Nicotine causes brain chemistry changes that effect mood. Once you stop the nicotine, it will take a while for your brain chemistry to go to its normal state. And hell, your brain chemistry hasn't been in its normal state since puberty started.
One smart fellow once asked me 'when has worrying about something ever made the outcome better?'. Answer: never. You are a young man: its time for you to stay quit, and start to knock off this excess worrying about things. Your worry doesn't make the outcome better, so find something else that does.