If you have a high news or social media diet, cut it back dramatically. It helps a lot for mood and just general worldview. Most of the problems on Twitter and in the news are not real problems that interfere with the average person's day to day life.

Keeping a notebook and writing down one good thing in it every morning helps a lot, too. Maybe something that went well for you the previous day, or something you're grateful for.

I know it's no fix for mental illness of a more "internal chemical imbalance" type, but every little bit helps.