One cigarette does NOT cancel the gains you have made.
Most people take more than one attempt before quitting.
It is important to remember that:
It might take multiple attempts before you’re able to successfully quit.
Don’t be disheartened.
The most important thing is to focus on the days you have successfully managed to avoid smoking.
Most people struggle with quitting.
Keep working on it.

What do you do next?
The following are the steps to getting back on course
Set a new quit date once you're ready to try again. Avoid postponing it for too long (try again in less than a month)
Revisit your quit plan, to remember why you decided to quit and see what can be improved.
Speak to a friend/your doctor if you feel you need more support to successfully quit. Counsellors are very good at helping you quit too.
Give it another go. Remember to be proud of every smoke free day you have.
There are medications that could help improve your abstinence even before your quit date. Speak to your doctor if you’re interested.