I can't make myself happy directly if I'm feeling down. It just doesn't work -- I end up just feeling lame that I'm artificially trying to cheer myself up as if I were disregarding the problems in my life.
What instead works for me is to redirect through some other form of intensity. Loud energetic (some might call it "angry") music sometimes works, sometimes playing a difficult video game works, sometimes just going and getting some physical activity works. Rhythm games do a good job of combining those categories; DanceDanceRevolution hits all three. Once I'm well and truly psyched up by whatever it is I'm doing, then I'm not
sad anymore and THEN I can calm down and get happy.
If my mood is such that I can't bring that kind of intensity to bear, then I go the polar opposite direction and redirect into something peaceful and calming.
"The Art of Zen" by Zircon is one of my favorite songs for that, but I have a whole Spotify playlist of stuff that's soothing enough to be listenable when I'm upset but energetic enough to lift my mood over time.
Meditation helps too, and this kind of music is GREAT for the kind of meditation that can pull you out of a negative mood: close your eyes, let the music fill your body, focus your awareness on the texture of the music and the feeling of your body reacting to it. If negative thoughts come to mind, acknowledge the thought and gently place it aside instead of getting upset that you can't get your mind off of it.