Actually smoking is more to your psychology rather than the nicotine itself. Nicotine will be cleansed from your blood within few days.

Even someone who stopped for few years, he/she might get back to smoking when their mind refreshed by those memories and habit. Such as, you enjoy smoking when having a drink with friends etc, you'll tend to miss it.

Hence, you need to delete all this memory CLEAN. Especially the memory of picking up a cigs when you're stress or depressed. You have to look for an alternative to fill up your time and something to release your stress.