Its ok to slip. You being happy right now proves that being depressed is not a permanent thing. So hold that thought for when you have a bad day because you know you can also have good days.

But also do not dwell on the inevitable bad days. They will come when they come. Enjoy the good days right now. Further more if you find yourself focusing mentally on if your happiness will fade then forcible think of something else or do something else. I used to have my wife play a game or two of tetris on her device when she felt that way.

There is no faster way to bring yourself down than to focus on what will happen when you are inevitably down. So you need a strategy to prevent that mindset.

The saying, pretend you are happy until you forget you are pretending, works both ways. You can force yourself to spiral down by focusing on the spiral. So anytime you feel you are doing that you need to break out of that mental process.