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. There will also be times when naturally you are feeling more anxious than others. 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”.