Stress is pressure we feel from events in daily life. It's an ancient defence mechanism that releases adrenaline into our body in preparation for fight or flight & in dangeraous situations is a life saver as it makes us stronger, faster, and able to do what needs to be done without conscious thought. The trouble is in modern times most of the stress we feel comes from pressure of work etc. Because most of us rely on work to pay the bills, to keep a roof over our heads & to feed ourselves, we can be under constant fear of losing our jobs, so losing the ablilty to keep ourselves and our families alive so the old mechanism kicks in as the subconscious mind interprets the situation in the same way.
Were you stresssed before you started smoking? You may or may not have been, I don't know your personal circumstances, but I'm pretty sure that when you become a non-smoker, you'll be less stressed than you are now by the challenges you face in your life. This is because instead of being some kind of escape, they're just an extra burden on an already stressed mind/body system, and possibly on your finances, work/life balance (you may have to work longer hours just to pay for the cigs - on a minimum wage someone would have to work an extra day to support the average smoking habit..
Lets just get that one out of the way: Nicotine is not an addictive drug. It has no medicinal purposes whatsoever, it's a poison. It's used in crop sprays and it's in a pesticide that's just been banned here that was killing bees. The perceived calming effect is not from the cigarette at all, all that's doing is triggering your body's defence mechanisms which release endorphins to combat the ill effects, you've trained your body to expect the next 'nicotine attack' at certain intervals, and saves your natural body painkillers and feelgood chemicals just for those occasions. Consequently it's difficult to get access to them at other times.. Remember your first fag? For most people that's the most unpleasant one. If it was a drug that would have made you feel nice as well.
Without them you can either work less or have more money. Less work = less stress. More money = less stress for most people. Without them your health recovers. Healthy body = less stress, ability to deal with situations with a clear mind & robust physicality. Without them you'll never worry about running out, not worrying about running out = less stress. Without them there's no need to stand in freezing doorways like a pariah while your colleages are enjoying their cup of tea in a warm office. Not standing in a freezing doorway feeling like a pariah = less stress. The list goes on.