That’s okay. It’s in the nature of the mind to wander off, drift off or lose focus. Being present takes lots of mindfulness practice.
When your mind wanders off, be okay with that and just bring your attention back to your visualization.
Alternatively, allow your mind to wander off and notice yourself as a non-smoker in those thoughts and situations.
For example, if your mind drifts off and starts thinking about other things like your job, relationships or responsibilities, take advantage of that and imagine yourself being a non-smoker in those situations.