Well, "habit" is just a kind of coy word for addiction :) 20 a day is addiction.
Generally accepted, I think ,that there are 2 elements to cigarette addiction - the physical / biochemical addiction to nicotine, which can usually be broken after 4-5 days.
The second element is the psychological - as you yourself have mentioned, smoking is often desribed as a kind of an emotional crutch. Often mentioned is the sense of relief from stress/anxiety that having a cigarette brings - but most of that relief comes from satisfying the bodies craving for nicotine, rather than helping with the actual stress or anxiety you are suffering from.
I gave up about 2.5 years ago now, and do not regret it at all , but I did it the wrong way - each time I felt the need for a cigarette, I would reach for a snack instead, hence putting on weight :) - If you can use something like a mint instead, or a piece of fruit, that would work much better :)
Regardless, it is probably better for you to be a bit overweight and to have to diet for a while, rather than continuing the damage to your bodies vital systems by continuing to smoke.....
And the improvement to the money in your pocket is enormous - a pack of 20 a day is what? around £6/day, or £42 /week or about £2,000 / year....