I think a lot of people fight their sadness and end up with more sadness. This is why mindfulness is so helpful, it allows you to accept that some days are going to be shit and that that is okay. Notice the happy times. Force yourself to do things that you should be enjoying. Listen to nice music, look at paintings, notice the trees, pet a cat. Force a smile. Sometimes, eventually, your joy in those things becomes genuine. And then things will get shit again because that is life. But that is okay, you start again, because that is all you can do