Apparently (I did a lot of research when I was quitting because it was all I could think of) you have receptors in your brain that increase in number when you intake nicotine...so the more/longer you smoke, the more you have these receptors, and everytime you have nicotine, they all light up and give you the happy feeling.
It takes up to 8 weeks for the extra receptors you've created through smoking to 'die'...for a few days they go nuts, you have awful cravings, mood differences etc. Then they start dulling, and it will just get easier and easier. When they're all gone, if you start smoking again it won't be down to any physical addiction.
If you're really struggling, I would also recommend an e cig. I quit 2 years ago cold turkey and lasted a few weeks. Quit again 18 months ago, with patches, but really really struggled...I was a complete bitch to everyone, I had night terrors (common side effect of patches), I lost my memory etc. Lasted 8 months.
I've now been using an e cig for 3 months, but I've never felt less like having a cigarette. Quitting was easy. No mood differences, no habit to break (I still go 'for a fag' with work colleagues, on a night out etc), and now i'm just working on slowly decreasing the nicotine in the e cigs.