Just because you've stopped smoking doesn't mean you can eat exactly what you did while you were smoking, for the simple reason that your body isn't the same now you've stopped.
You need to hold on to those good eating habits you developed when you first gave up. If you give in and hit the chocolate (and the biscuits, sweets and cakes) once in a while, congratulations - you're human! But don't let one binge spiral out of control: make up for it and make yourself feel better about the odd binge by having a super-healthy meal the next time you eat: 1 hard-boiled egg, raw vegetables and a low-fat yogourt. You'll soon be back on the wagon, so to speak.
Now that your lungs are no longer having to cope with cigarettes, you can start to enjoy sport: it's a far better addiction to have, and you're in a better physical condition to enjoy exercise without collapsing in a heap after 5 minutes. It's also really good for lifting your spirits and staying in shape. Try and spend 2 - 3 hours a week doing sport, and don't forget to walk!