Depression is a problem for some people at this time of year. Look after yourself and those you around you too.
Here are some DIY things to try. Being consistent in self-disciplining my thoughts and taking supplements has always helped bring me out of a depressed state. Sometimes it took days and other times a few weeks. If self-help does not make an improvement, please see a doctor. There is no shame in taking care of your health!
There are more self-help tips on the internet, but these should give you a place to start.