Relapse
A slip isn't a failure, and it isn't back to zero.
Most people who successfully stop have tried more than once. That's not the exception. It's how quitting usually works. If you slip, the single most important thing is what you tell yourself next, because "I've blown it, I'm useless, might as well carry on" does far more damage than the slip itself. A slip is information, not a verdict.
A slip is not a relapse
It's worth separating two things. A slip is one lapse: a joint at a party, a rough night you caved on. A relapse is sliding back into regular use. The danger is letting the first become the second, and that transition happens almost entirely in your head. It's the "well, that's ruined it" thought that turns one evening into a fortnight. One slip does not undo the progress your body and brain have already made.
What to do after a slip
Don't spiral. Reset. The next choice is the only one that matters. You haven't gone back to day zero. You've had a lapse in a process that's still moving forward.
Get curious about it. What was the trigger? Stress, a particular person, a particular place, being caught off guard with no plan? A slip points straight at the gap in your defences. Patch that gap and you're stronger than before it happened.
Be as kind to yourself as you'd be to a friend. Self-compassion isn't going soft. The research on behaviour change is clear that shame and self-punishment make relapse more likely, not less. Someone who forgives the slip and gets straight back to it does better than someone who beats themselves up.
Learn, adjust, continue. Maybe you need a firmer plan for evenings, or to avoid a certain situation for a while, or to lean on more support than you thought. Adjust the plan and carry on. Every attempt teaches you something the next one uses.
Quitting isn't a single clean line for most people. It's a series of attempts that gradually add up. Judge yourself on the direction of travel, not on a single bad night.