We weren't born knowing bladder and bowel control. We learned it by practicing.
We weren't born knowing how to walk. We had to learn that.
Thumbsucking was pure instinct but something we had to unlearn.
Perhaps smoking was a replacement for thumbsucking? A sense of security with a hit of dopamine?:
Thumbsucking won't kill you but smoking will.
You can unlearn it just like you did thumbsucking.
The further you can distance yourself from those hand to mouth motions and the memories that drive them, the better off you are.
Unlearning the habit part of smoking and the time spent doing that is the key to your success.