Slip-ups will happen — everybody makes mistakes, and it doesn’t mean that you have failed completely.
You can still stop at one drink, one cone or one hit, and go from there - you can start with a clean slate.
Also, with depression, realise that your mood will sometimes be lower than at other times. It doesn’t mean you are getting worse, or headed for a relapse, it just means that you are experiencing what everybody does — a simple change in mood, that won’t last forever.
But, if you have a slip-up, it is more likely to turn into a full-blown relapse if you give into the “breaking the rule effect”.